Methods and apparatus for driving electro-optic displays

ABSTRACT

Waveforms for driving electro-optic displays, especially bistable electro-optic displays, are modified by one or more of insertion of at least one balanced pulse pair into a base waveform; excision of at least one balanced pulse pair from the base waveform; and insertion of at least one period of zero voltage into the base waveform. Such modifications permit fine control of gray levels.

REFERENCE TO RELATED APPLICATIONS

This application is a division of copending application Ser. No.11/161,715, filed Aug. 13, 2005 (Publication No. 2005/0280626), whichclaims benefit of the following provisional Applications: (a)Application Ser. No. 60/601,242, filed Aug. 13, 2004; (b) ApplicationSer. No. 60/522,372, filed Sep. 21, 2004; and (c) Application Ser. No.60/522,393, filed Sep. 24, 2004.

The aforementioned copending application Ser. No. 11/161,715 is also acontinuation-in-part of application Ser. No. 10/904,707, filed Nov. 24,2004 (Publication No. 2005/0179642), which itself claims benefit ofprovisional Application Ser. Nos. 60/481,711 and 60/481,713, both filedNov. 26, 2003.

The aforementioned application Ser. No. 10/904,707 is acontinuation-in-part of application Ser. No. 10/879,335, filed Jun. 29,2004 (Publication No. 2005/0024353, now U.S. Pat. No. 7,528,822, issuedMay 5, 2009), which claims benefit of the following provisionalApplications: Ser. No. 60/481,040, filed Jun. 30, 2003; Ser. No.60/481,053, filed Jul. 2, 2003; and Ser. No. 60/481,405, filed Sep. 23,2003.

The aforementioned application Ser. No. 10/879,335 is also acontinuation-in-part of application Ser. No. 10/814,205, filed Mar. 31,2004 (Publication No. 2005/0001812, now U.S. Pat. No. 7,119,772, issuedOct. 10, 2006), which claims benefit of the following provisionalApplications: Ser. No. 60/320,070, filed Mar. 31, 2003; Ser. No.60/320,207, filed May 5, 2003; Ser. No. 60/481,669, filed Nov. 19, 2003;Ser. No. 60/481,675, filed Nov. 20, 2003; and Ser. No. 60/557,094, filedMar. 26, 2004.

The aforementioned application Ser. No. 10/814,205 is related toapplication Ser. No. 10/249,973, filed May 23, 2003 (Publication No.2005/0270261, now U.S. Pat. No. 7,193,625, issued Mar. 20, 2007), whichclaims benefit of provisional Application Ser. Nos. 60/319,315, filedJun. 13, 2002 and Ser. No. 60/319,321, filed Jun. 18, 2002.

The aforementioned application Ser. No. 10/249,973 is also acontinuation-in-part of application Ser. No. 10/065,795, filed Nov. 20,2002 (Publication No. 2003/0137521, now U.S. Pat. No. 7,012,600, Mar.14, 2006), which itself claims benefit of the following provisionalApplications: Ser. No. 60/319,007, filed Nov. 20, 2001; Ser. No.60/319,010, filed Nov. 21, 2001; Ser. No. 60/319,034, filed Dec. 18,2001; Ser. No. 60/319,037, filed Dec. 20, 2001; and Ser. No. 60/319,040,filed Dec. 21, 2001.

This application is also related to application Ser. No. 10/063,236,filed Apr. 2, 2002 (Publication No. 2002/0180687, now U.S. Pat. No.7,170,670); application Ser. No. 10/064,279, filed Jun. 28, 2002 (nowU.S. Pat. No. 6,657,772); application Ser. No. 10/064,389, filed Jul. 9,2002 (Publication No. 2003/0025855, now U.S. Pat. No. 6,831,769); andapplication Ser. No. 10/249,957, filed May 22, 2003 (Publication No.2004/0027327, now U.S. Pat. No. 6,982,178).

The aforementioned application Ser. Nos. 10/904,707; 10/879,335;10/814,205; 10/249,973; and 10/065,795 may hereinafter for conveniencecollectively be referred to as the “MEDEOD” (MEthods for DrivingElectro-Optic Displays) applications.

The entire contents of these copending applications, and of all otherU.S. patents and published and copending applications mentioned below,are herein incorporated by reference.

BACKGROUND OF INVENTION

This invention relates to methods for driving electro-optic displays,especially bistable electro-optic displays, and to apparatus(controllers) for use in such methods. More specifically, this inventionrelates to driving methods which are intended to enable more accuratecontrol of gray states of the pixels of an electro-optic display. Thisinvention also relates to driving methods which are intended to enablesuch displays to be driven in a manner which allows compensation for the“dwell time” during which a pixel has remained in a particular opticalstate prior to a transition, while still allowing the drive scheme usedto drive the display to be DC balanced. This invention is especially,but not exclusively, intended for use with particle-basedelectrophoretic displays in which one or more types of electricallycharged particles are suspended in a liquid and are moved through theliquid under the influence of an electric field to change the appearanceof the display.

The electro-optic displays in which the methods of the present inventionare used often contain an electro-optic material which is a solid in thesense that the electro-optic material has solid external surfaces,although the material may, and often does, have internal liquid- orgas-filled space. Such displays using solid electro-optic materials mayhereinafter for convenience be referred to as “solid electro-opticdisplays”.

The term “electro-optic” as applied to a material or a display, is usedherein in its conventional meaning in the imaging art to refer to amaterial having first and second display states differing in at leastone optical property, the material being changed from its first to itssecond display state by application of an electric field to thematerial. Although the optical property is typically color perceptibleto the human eye, it may be another optical property, such as opticaltransmission, reflectance, luminescence or, in the case of displaysintended for machine reading, pseudo-color in the sense of a change inreflectance of electromagnetic wavelengths outside the visible range.

The term “gray state” is used herein in its conventional meaning in theimaging art to refer to a state intermediate two extreme optical statesof a pixel, and does not necessarily imply a black-white transitionbetween these two extreme states. For example, several of the patentsand published applications referred to below describe electrophoreticdisplays in which the extreme states are white and deep blue, so that anintermediate “gray state” would actually be pale blue. Indeed, asalready mentioned the transition between the two extreme states may notbe a color change at all. The term “gray level” is used herein to denotethe possible optical states of a pixel, including the two extremeoptical states.

The terms “bistable” and “bistability” are used herein in theirconventional meaning in the art to refer to displays comprising displayelements having first and second display states differing in at leastone optical property, and such that after any given element has beendriven, by means of an addressing pulse of finite duration, to assumeeither its first or second display state, after the addressing pulse hasterminated, that state will persist for at least several times, forexample at least four times, the minimum duration of the addressingpulse required to change the state of the display element. It is shownin published U.S. Patent Application No. 2002/0180687 that someparticle-based electrophoretic displays capable of gray scale are stablenot only in their extreme black and white states but also in theirintermediate gray states, and the same is true of some other types ofelectro-optic displays. This type of display is properly called“multi-stable” rather than bistable, although for convenience the term“bistable” may be used herein to cover both bistable and multi-stabledisplays.

The term “impulse” is used herein in its conventional meaning of theintegral of voltage with respect to time. However, some bistableelectro-optic media act as charge transducers, and with such media analternative definition of impulse, namely the integral of current overtime (which is equal to the total charge applied) may be used. Theappropriate definition of impulse should be used, depending on whetherthe medium acts as a voltage-time impulse transducer or a charge impulsetransducer.

Much of the discussion below will focus on methods for driving one ormore pixels of an electro-optic display through a transition from aninitial gray level to a final gray level (which may or may not bedifferent from the initial gray level). The term “waveform” will be usedto denote the entire voltage against time curve used to effect thetransition from one specific initial gray level to a specific final graylevel. Typically, as illustrated below, such a waveform will comprise aplurality of waveform elements; where these elements are essentiallyrectangular (i.e., there a given element comprises application of aconstant voltage for a period of time), the elements may be called“voltage pulses” or “drive pulses”. The term “drive scheme” denotes aset of waveforms sufficient to effect all possible transitions betweengray levels for a specific display.

Several types of electro-optic displays are known. One type ofelectro-optic display is a rotating bichromal member type as described,for example, in U.S. Pat. Nos. 5,808,783; 5,777,782; 5,760,761;6,054,071 6,055,091; 6,097,531; 6,128,124; 6,137,467; and 6,147,791(although this type of display is often referred to as a “rotatingbichromal ball” display, the term “rotating bichromal member” ispreferred as more accurate since in some of the patents mentioned abovethe rotating members are not spherical). Such a display uses a largenumber of small bodies (typically spherical or cylindrical) which havetwo or more sections with differing optical characteristics, and aninternal dipole. These bodies are suspended within liquid-filledvacuoles within a matrix, the vacuoles being filled with liquid so thatthe bodies are free to rotate. The appearance of the display is changedto applying an electric field thereto, thus rotating the bodies tovarious positions and varying which of the sections of the bodies isseen through a viewing surface. This type of electro-optic medium istypically bistable.

Another type of electro-optic display uses an electrochromic medium, forexample an electrochromic medium in the form of a nanochromic filmcomprising an electrode formed at least in part from a semi-conductingmetal oxide and a plurality of dye molecules capable of reversible colorchange attached to the electrode; see, for example O'Regan, B., et al.,Nature 1991, 353, 737; and Wood, D., Information Display, 18(3), 24(March 2002). See also Bach, U., et al., Adv. Mater., 2002, 14(11), 845.Nanochromic films of this type are also described, for example, in U.S.Pat. No. 6,301,038, International Application Publication No. WO01/27690, and in U.S. Patent Application 2003/0214695. This type ofmedium is also typically bistable.

Another type of electro-optic display, which has been the subject ofintense research and development for a number of years, is theparticle-based electrophoretic display, in which a plurality of chargedparticles move through a fluid under the influence of an electric field.Electrophoretic displays can have attributes of good brightness andcontrast, wide viewing angles, state bistability, and low powerconsumption when compared with liquid crystal displays. Nevertheless,problems with the long-term image quality of these displays haveprevented their widespread usage. For example, particles that make upelectrophoretic displays tend to settle, resulting in inadequateservice-life for these displays.

As noted above, electrophoretic media require the presence of a fluid.In most prior art electrophoretic media, this fluid is a liquid, butelectrophoretic media can be produced using gaseous fluids; see, forexample, Kitamura, T., et al., “Electrical toner movement for electronicpaper-like display”, IDW Japan, 2001, Paper HCS1-1, and Yamaguchi, Y.,et al., “Toner display using insulative particles chargedtriboelectrically”, IDW Japan, 2001, Paper AMD4-4). See also EuropeanPatent Applications 1,429,178; 1,462,847; and 1,482,354; andInternational Applications WO 2004/090626; WO 2004/079442; WO2004/077140; WO 2004/059379; WO 2004/055586; WO 2004/008239; WO2004/006006; WO 2004/001498; WO 03/091799; and WO 03/088495. Suchgas-based electrophoretic media appear to be susceptible to the sametypes of problems due to particle settling as liquid-basedelectrophoretic media, when the media are used in an orientation whichpermits such settling, for example in a sign where the medium isdisposed in a vertical plane. Indeed, particle settling appears to be amore serious problem in gas-based electrophoretic media than inliquid-based ones, since the lower viscosity of gaseous fluids ascompared with liquid ones allows more rapid settling of theelectrophoretic particles.

Numerous patents and applications assigned to or in the names of theMassachusetts Institute of Technology (MIT) and E Ink Corporation haverecently been published describing encapsulated electrophoretic media.Such encapsulated media comprise numerous small capsules, each of whichitself comprises an internal phase containing electrophoretically-mobileparticles suspended in a fluid, and a capsule wall surrounding theinternal phase. Typically, the capsules are themselves held within apolymeric binder to form a coherent layer positioned between twoelectrodes. Encapsulated media of this type are described, for example,in U.S. Pat. Nos. 5,930,026; 5,961,804; 6,017,584; 6,067,185; 6,118,426;6,120,588; 6,120,839; 6,124,851; 6,130,773; 6,130,774; 6,172,798;6,177,921; 6,232,950; 6,249,271; 6,252,564; 6,262,706; 6,262,833;6,300,932; 6,312,304; 6,312,971; 6,323,989; 6,327,072; 6,376,828;6,377,387; 6,392,785; 6,392,786; 6,413,790; 6,422,687; 6,445,374;6,445,489; 6,459,418; 6,473,072; 6,480,182; 6,498,114; 6,504,524;6,506,438; 6,512,354; 6,515,649; 6,518,949; 6,521,489; 6,531,997;6,535,197; 6,538,801; 6,545,291; 6,580,545; 6,639,578; 6,652,075;6,657,772; 6,664,944; 6,680,725; 6,683,333; 6,704,133; 6,710,540;6,721,083; 6,724,519; 6,727,881; 6,738,050; 6,750,473; 6,753,999;6,816,147; 6,819,471; 6,822,782; 6,825,068; 6,825,829; 6,825,970;6,831,769; 6,839,158; 6,842,167; 6,842,279; 6,842,657; 6,864,875;6,865,010; 6,866,760; 6,870,661; 6,900,851; and 6,922,276; and U.S.Patent Applications Publication Nos. 2002/0060321; 2002/0063661;2002/0090980; 2002/0113770; 2002/0130832; 2002/0180687; 2003/0011560;2003/0020844; 2003/0025855; 2003/0102858; 2003/0132908; 2003/0137521;2003/0214695; 2003/0222315; 2004/0012839; 2004/0014265; 2004/0027327;2004/0075634; 2004/0094422; 2004/0105036; 2004/0112750; 2004/0119681;2004/0136048; 2004/0155857; 2004/0180476; 2004/0190114; 2004/0196215;2004/0226820; 2004/0239614; 2004/0252360; 2004/0257635; 2004/0263947;2005/0000813; 2005/0001812; 2005/0007336; 2005/0007653; 2005/0012980;2005/0017944; 2005/0018273; 2005/0024353; 2005/0035941; 2005/0041004;2005/0062714; 2005/0067656; 2005/0078099; 2005/0105159; 2005/0122284;2005/0122306; 2005/0122563; 2005/0122564; 2005/0122565; 2005/0151709;and 2005/0152022; and International Applications Publication Nos. WO99/67678; WO 00/05704; WO 00/38000; WO 00/36560; WO 00/67110; WO00/67327; WO 01/07961; and WO 03/107,315.

Many of the aforementioned patents and applications recognize that thewalls surrounding the discrete microcapsules in an encapsulatedelectrophoretic medium could be replaced by a continuous phase, thusproducing a so-called “polymer-dispersed electrophoretic display” inwhich the electrophoretic medium comprises a plurality of discretedroplets of an electrophoretic fluid and a continuous phase of apolymeric material, and that the discrete droplets of electrophoreticfluid within such a polymer-dispersed electrophoretic display may beregarded as capsules or microcapsules even though no discrete capsulemembrane is associated with each individual droplet; see for example,the aforementioned U.S. Pat. No. 6,866,760. Accordingly, for purposes ofthe present application, such polymer-dispersed electrophoretic mediaare regarded as sub-species of encapsulated electrophoretic media.

An encapsulated electrophoretic display typically does not suffer fromthe clustering and settling failure mode of traditional electrophoreticdevices and provides further advantages, such as the ability to print orcoat the display on a wide variety of flexible and rigid substrates.(Use of the word “printing” is intended to include all forms of printingand coating, including, but without limitation: pre-metered coatingssuch as patch die coating, slot or extrusion coating, slide or cascadecoating, curtain coating; roll coating such as knife over roll coating,forward and reverse roll coating; gravure coating; dip coating; spraycoating; meniscus coating; spin coating; brush coating; air knifecoating; silk screen printing processes; electrostatic printingprocesses; thermal printing processes; ink jet printing processes; andother similar techniques.) Thus, the resulting display can be flexible.Further, because the display medium can be printed (using a variety ofmethods), the display itself can be made inexpensively.

A related type of electrophoretic display is a so-called “microcellelectrophoretic display”. In a microcell electrophoretic display, thecharged particles and the fluid are not encapsulated within capsules butinstead are retained within a plurality of cavities formed within acarrier medium, typically a polymeric film. See, for example,International Application Publication No. WO 02/01281, and U.S. PatentApplication Publication No. 2002/0075556, both assigned to SipixImaging, Inc.

Other types of electro-optic media may also be used in the displays ofthe present invention.

Although electrophoretic media are often opaque (since, for example, inmany electrophoretic media, the particles substantially blocktransmission of visible light through the display) and operate in areflective mode, many electrophoretic displays can be made to operate ina so-called “shutter mode” in which one display state is substantiallyopaque and one is light-transmissive. See, for example, theaforementioned U.S. Pat. Nos. 6,130,774 and 6,172,798, and U.S. Pat.Nos. 5,872,552; 6,144,361; 6,271,823; 6,225,971; and 6,184,856.Dielectrophoretic displays, which are similar to electrophoreticdisplays but rely upon variations in electric field strength, canoperate in a similar mode; see U.S. Pat. No. 4,418,346.

The bistable or multi-stable behavior of particle-based electrophoreticdisplays, and other electro-optic displays displaying similar behavior(such displays may hereinafter for convenience be referred to as“impulse driven displays”), is in marked contrast to that ofconventional liquid crystal (“LC”) displays. Twisted nematic liquidcrystals act are not bi- or multi-stable but act as voltage transducers,so that applying a given electric field to a pixel of such a displayproduces a specific gray level at the pixel, regardless of the graylevel previously present at the pixel. Furthermore, LC displays are onlydriven in one direction (from non-transmissive or “dark” to transmissiveor “light”), the reverse transition from a lighter state to a darker onebeing effected by reducing or eliminating the electric field. Finally,the gray level of a pixel of an LC display is not sensitive to thepolarity of the electric field, only to its magnitude, and indeed fortechnical reasons commercial LC displays usually reverse the polarity ofthe driving field at frequent intervals. In contrast, bistableelectro-optic displays act, to a first approximation, as impulsetransducers, so that the final state of a pixel depends not only uponthe electric field applied and the time for which this field is applied,but also upon the state of the pixel prior to the application of theelectric field.

Whether or not the electro-optic medium used is bistable, to obtain ahigh-resolution display, individual pixels of a display must beaddressable without interference from adjacent pixels. One way toachieve this objective is to provide an array of non-linear elements,such as transistors or diodes, with at least one non-linear elementassociated with each pixel, to produce an “active matrix” display. Anaddressing or pixel electrode, which addresses one pixel, is connectedto an appropriate voltage source through the associated non-linearelement. Typically, when the non-linear element is a transistor, thepixel electrode is connected to the drain of the transistor, and thisarrangement will be assumed in the following description, although it isessentially arbitrary and the pixel electrode could be connected to thesource of the transistor. Conventionally, in high resolution arrays, thepixels are arranged in a two-dimensional array of rows and columns, suchthat any specific pixel is uniquely defined by the intersection of onespecified row and one specified column. The sources of all thetransistors in each column are connected to a single column electrode,while the gates of all the transistors in each row are connected to asingle row electrode; again the assignment of sources to rows and gatesto columns is conventional but essentially arbitrary, and could bereversed if desired. The row electrodes are connected to a row driver,which essentially ensures that at any given moment only one row isselected, i.e., that there is applied to the selected row electrode avoltage such as to ensure that all the transistors in the selected roware conductive, while there is applied to all other rows a voltage suchas to ensure that all the transistors in these non-selected rows remainnon-conductive. The column electrodes are connected to column drivers,which place upon the various column electrodes voltages selected todrive the pixels in the selected row to their desired optical states.(The aforementioned voltages are relative to a common front electrodewhich is conventionally provided on the opposed side of theelectro-optic medium from the non-linear array and extends across thewhole display.) After a pre-selected interval known as the “line addresstime” the selected row is deselected, the next row is selected, and thevoltages on the column drivers are changed to that the next line of thedisplay is written. This process is repeated so that the entire displayis written in a row-by-row manner.

It might at first appear that the ideal method for addressing such animpulse-driven electro-optic display would be so-called “generalgrayscale image flow” in which a controller arranges each writing of animage so that each pixel transitions directly from its initial graylevel to its final gray level. However, inevitably there is some errorin writing images on an impulse-driven display. Some such errorsencountered in practice include:

-   -   (a) Prior State Dependence; With at least some electro-optic        media, the impulse required to switch a pixel to a new optical        state depends not only on the current and desired optical state,        but also on the previous optical states of the pixel.    -   (b) Dwell Time Dependence; With at least some electro-optic        media, the impulse required to switch a pixel to a new optical        state depends on the time that the pixel has spent in its        various optical states. The precise nature of this dependence is        not well understood, but in general, more impulse is required        that longer the pixel has been in its current optical state.    -   (c) Temperature Dependence; The impulse required to switch a        pixel to a new optical state depends heavily on temperature.    -   (d) Humidity Dependence; The impulse required to switch a pixel        to a new optical state depends, with at least some types of        electro-optic media, on the ambient humidity.    -   (e) Mechanical Uniformity; The impulse required to switch a        pixel to a new optical state may be affected by mechanical        variations in the display, for example variations in the        thickness of an electro-optic medium or an associated lamination        adhesive. Other types of mechanical non-uniformity may arise        from inevitable variations between different manufacturing        batches of medium, manufacturing tolerances and materials        variations.    -   (f) Voltage Errors; The actual impulse applied to a pixel will        inevitably differ slightly from that theoretically applied        because of unavoidable slight errors in the voltages delivered        by drivers.

General grayscale image flow suffers from an “accumulation of errors”phenomenon. For example, imagine that temperature dependence results ina 0.2 L* (where L* has the usual CIE definition:L*=116(R/R ₀)^(1/3)−16,where R is the reflectance and R0 is a standard reflectance value) errorin the positive direction on each transition. After fifty transitions,this error will accumulate to 10 L*. Perhaps more realistically, supposethat the average error on each transition, expressed in terms of thedifference between the theoretical and the actual reflectance of thedisplay is ±0.2 L*. After 100 successive transitions, the pixels willdisplay an average deviation from their expected state of 2 L*; suchdeviations are apparent to the average observer on certain types ofimages.

This accumulation of errors phenomenon applies not only to errors due totemperature, but also to errors of all the types listed above. Asdescribed in the aforementioned 2003/0137521, compensating for sucherrors is possible, but only to a limited degree of precision. Forexample, temperature errors can be compensated by using a temperaturesensor and a lookup table, but the temperature sensor has a limitedresolution and may read a temperature slightly different from that ofthe electro-optic medium. Similarly, prior state dependence can becompensated by storing the prior states and using a multi-dimensionaltransition matrix, but controller memory limits the number of statesthat can be recorded and the size of the transition matrix that can bestored, placing a limit on the precision of this type of compensation.

Thus, general grayscale image flow requires very precise control ofapplied impulse to give good results, and empirically it has been foundthat, in the present state of the technology of electro-optic displays,general grayscale image flow is infeasible in a commercial display.

Almost all electro-optic medium have a built-in resetting (errorlimiting) mechanism, namely their extreme (typically black and white)optical states, which function as “optical rails”. After a specificimpulse has been applied to a pixel of an electro-optic display, thatpixel cannot get any whiter (or blacker). For example, in anencapsulated electrophoretic display, after a specific impulse has beenapplied, all the electrophoretic particles are forced against oneanother or against the capsule wall, and cannot move further, thusproducing a limiting optical state or optical rail. Because there is adistribution of electrophoretic particle sizes and charges in such amedium, some particles hit the rails before others, creating a “softrails” phenomenon, whereby the impulse precision required is reducedwhen the final optical state of a transition approaches the extremeblack and white states, whereas the optical precision required increasesdramatically in transitions ending near the middle of the optical rangeof the pixel.

Various types of drive schemes for electro-optic displays are knownwhich take advantage of optical rails. For example, FIGS. 9 and 10 ofthe aforementioned 2003/0137521, and the related description atParagraphs [0177] to [0180], describe a “slide show” drive scheme inwhich the entire display is driven to at least one optical rail beforeany new image is written. Obviously, a pure general grayscale image flowdrive scheme cannot rely upon using the optical rails to prevent errorsin gray levels since in such a drive scheme any given pixel can undergoan infinitely large number of changes in gray level without evertouching either optical rail.

Before proceeding further, it is desirable to define slideshow driveschemes more precisely. The fundamental slideshow drive scheme is that atransition from an initial optical state (gray level) to a final(desired) optical state (gray level) is achieved by making transitionsto a finite number of intermediate states, where the minimum number ofintermediate states is one. Preferably, the intermediate states are ator near the extreme states of the electro-optic medium used. Thetransitions will differ from pixel to pixel in a display, because theydepend upon the initial and final optical states. The waveform for aspecific transition for a given pixel of a display may be expressed as:R ₂

goal₁

goal₂

. . .

goal_(n)

R ₁  (Scheme 1)where there is at least one intermediate or goal state between theinitial state R₂ and the final state R₁. The goal states are, ingeneral, functions of the initial and final optical states. Thepresently preferred number of intermediate states is two, but more orfewer intermediate states may be used. Each of the individualtransitions within the overall transition is achieved using a waveformelement (typically a voltage pulse) sufficient to drive the pixel fromone state of the sequence to the next state. For example, in thewaveform indicated symbolically above, the transition from R₂ to goal₁is typically achieved with a waveform element or voltage pulse. Thiswaveform element may be of a single voltage for a finite time (i.e., asingle voltage pulse), or may include a variety of voltages so that aprecise goal₁ state is achieved. This waveform element is followed by asecond waveform element to achieve the transition from goal₁ to goal₂.If only two goal states are used, the second waveform element isfollowed by a third waveform element that drives the pixel from thegoal₂ state to the final optical state R₁. The goal states may beindependent of both R₂ and R₁, or may depend upon one or both.

This invention seeks to provide improved slide show drive schemes forelectro-optic displays which achieve improved control of gray levels.This invention is particularly, although not exclusively, intended foruse in pulse width modulated drive schemes in which the voltage appliedto any given pixel of a display at any given moment can only be −V, 0 or+V, where V is an arbitrary voltage. More specifically, this inventionrelates to two distinct types of improvements in slide show driveschemes, namely (a) insertion of certain modifying elements into basewaveforms for such a drive scheme; and (b) arranging the drive scheme sothat at least certain gray levels are approached from the optical railfurther from the desired gray level.

In another aspect, this invention relates to dwell time compensation indrive schemes for electro-optic displays. As discussed in the MEDEODapplications, it has been found, at least in the case of manyparticle-based electro-optic displays, that the impulses necessary tochange a given pixel through equal changes in gray level (as judged byeye or by standard optical instruments) are not necessarily constant,nor are they necessarily commutative. For example, consider a display inwhich each pixel can display gray levels of 0 (white), 1, 2 or 3(black), beneficially spaced apart. (The spacing between the levels maybe linear in percentage reflectance, as measured by eye or byinstruments but other spacings may also be used. For example, thespacings may be linear in L* or may be selected to provide a specificgamma; a gamma of 2.2 is often adopted for monitors, and whenelectro-optic displays are be used as a replacement for monitors, use ofa similar gamma may be desirable.) It has been found that the impulsenecessary to change the pixel from level 0 to level 1 (hereinafter forconvenience referred to as a “0-1 transition”) is often not the same asthat required for a 1-2 or 2-3 transition. Furthermore, the impulseneeded for a 1-0 transition is not necessarily the same as the reverseof that needed for a 0-1 transition. In addition, some systems appear todisplay a “memory” effect, such that the impulse needed for (say) a 0-1transition varies somewhat depending upon whether a particular pixelundergoes 0-0-1, 1-0-1 or 3-0-1 transitions. (Where, the notation“x-y-z”, where x, y, and z are all optical states 0, 1, 2, or 3 denotesa sequence of optical states visited sequentially in time.) Althoughthese problems can be reduced or overcome by driving all pixels of thedisplay to one of the extreme states for a substantial period beforedriving the required pixels to other states, the resultant “flash” ofsolid color is often unacceptable; for example, a reader of anelectronic book may desire the text of the book to scroll down thescreen, and may be distracted, or lose his place, if the display isrequired to flash solid black or white at frequent intervals.Furthermore, such flashing of the display increases its energyconsumption and may reduce the working lifetime of the display. Finally,it has been found that, at least in some cases, the impulse required fora particular transition is affected by the temperature and the totaloperating time of the display, and that compensating for these factorsis desirable to secure accurate gray scale rendition.

As briefly mentioned above, it has been found that, at least in somecases, the impulse necessary for a given transition in a bistableelectro-optic display varies with the residence time of a pixel in itsoptical state, this phenomenon hereinafter being referred to as “dwelltime dependence” or “DTD”, although the term “dwell time sensitivity”was used in the aforementioned Application Ser. No. 60/320,070. Thus, itmay be desirable or even in some cases in practice necessary, to varythe impulse applied for a given transition as a function of the dwelltime of the pixel in its initial optical state.

The phenomenon of dwell time dependence will now be explained in moredetail with reference to FIG. 1 of the accompanying drawings, whichshows the reflectance of a pixel a function of time for a sequence oftransitions denoted R₃→R₂→R₁, where (generalizing the nomenclature usedabove) each of the R_(k) terms indicates a gray level in a sequence ofgray levels, with R's with larger indices occurring before R's withsmaller indices. The transitions between R₃ and R₂ and between R₂ and R₁are also indicated. DTD is the variation of the final optical state R₁caused by variation in the time spent in the optical state R₂, referredto as the dwell time. One can compensate for DTD by choosing differentwaveforms for different dwell times or different ranges of dwell timesin the previous optical state. This method of compensation is called“dwell-time compensation,” “DTC”, or simply “time compensation”.

However, such DTC may conflict with other desirable properties of driveschemes. In particular, for reasons discussed in detail in the MEDEODapplications, with many electro-optic displays it is highly desirable toensure that the drive scheme used is direct current (DC) balanced, inthe sense that, for any arbitrary series of transitions beginning andending in the same optical state, the applied impulse (i.e., theintegral of the applied voltage with respect to time) is zero. Thisguarantees that the net impulse (also called “DC imbalance”) experiencedby any pixel of the display is bounded by a known value regardless ofthe exact series of transitions undergone by that pixel. For example, a15 V, 300 msec pulse may be used to drive a pixel from a white to ablack state. After this transition, the pixel has experienced 4.5 V secof DC imbalance impulse. If a −15 V, 300 msec pulse is used to drive thepixel back to white, then the pixel is DC balanced for the overallexcursion from white to black and back to white. This DC balance shouldhold for all possible excursions from one original optical state, to aseries of optical states the same as or different from the originaloptical state, then back to the original optical state.

A drive scheme can be dwell-time-compensated by adding or removingvoltage features to or from a base drive scheme. For example, one mightbegin with a drive scheme for a two optical state (black and white)display, the drive scheme including the following four waveforms:

TABLE 1 Transition Waveform black to black 0 V for 420 msec black towhite −15 V for 400 msec, then 0 V for 20 msec white to black +15 V for400 msec, then 0 V for 20 msec white to white 0 V for 420 msec

This drive scheme is DC balanced, because any series of transitions thatbrings a pixel back to its initial optical state is DC balanced, thatis, the net area under the voltage profile for the entire series oftransitions is zero.

Optical errors can arise from DTD of a display. For example, a pixel maycan start in the white state, drive to the black state, dwell for atime, and then drive back to the white state. The final white statereflectance is a function of the time spent in the black state.

It is desirable to have a very small DTD. If this is not possible for aspecific electro-optic display, it is desirable to compensate for DTD,in accordance with one aspect of the present invention, by selectingdifferent waveforms for different ranges of dwell times in the prioroptical state. For example, one may find that the final white state inthe example just given is brighter after short dwell times in theprevious black state than after long dwell times in the previous blackstate. One dwell-time-compensation scheme would be to modify theduration of the pulse that brings the pixel layer from black to white tocounteract this DTD of the final optical state. For example, one couldshorten the pulse length in the black-to-white transition when the dwelltime in the previous black state is short, and keep the pulse longer forlong dwell times in the previous black state. This tends to produce adarker white state for shorter prior-state dwell times, whichcounteracts the effects of DTD. For example, one could choose ablack-to-white waveform that varies with dwell time in the black stateaccording to Table 2 below.

TABLE 2 Dwell time Waveform 0 to 0.3 sec −15 V for 280 msec, 0 V for 140msec 0.3 sec to 1 sec −15 V for 340 msec, 0 V for 80 msec 1 sec to 3 sec−15 V for 380 msec, 0 V for 40 msec 3 sec or greater −15 V for 400 msec,0 V for 20 msec

The problem with this approach to DTC of a drive scheme is that thedrive scheme as a whole is no longer DC balanced. Because the impulsefor a black-to-white transition is a function of the time spent in theblack state, and similarly the impulse for a white-to-black transitionmay be a function of the dwell time in the white state, the net impulseover a black-to-white-to-black sequence is, in general, not DC balanced.For example, suppose this sequence is carried out with a black-to-whitetransition after a short dwell time in black using a voltage pulse of−15 V for 280 msec=−4.2 V sec impulse, followed, after a long dwell inthe white state, by a white-to-black transition using a voltage pulse of15 V for 400 msec, for an impulse of 6 V sec. The net impulse in thissequence (black-white-black loop) is −4.2 V sec+6 V sec=1.8 V sec.Repeating this loop causes a build-up of DC imbalance, which can bedetrimental to the performance of the display.

Thus, this aspect of the present invention provides a method for dwelltime compensation of a DC balanced waveform or drive scheme thatpreserves the DC balance of the waveform or drive scheme.

Another aspect of the present invention relates to methods and apparatusfor driving electro-optic displays which permits rapid response to userinput. The aforementioned MEDEOD applications describe several methodsand controllers for driving electro-optic displays. Most of thesemethods and controllers make use of a memory having two image buffers,the first of which stores a first or initial image (present on thedisplay at the beginning of a transition or rewriting of the display)and the second of which stores a final image, which it desired to placeupon the display after the rewrite. The controller compares the initialand final images and, if they differ, applies to the various pixels ofthe display driving voltages which cause the pixels to undergo changesin optical state such that at the end of the rewrite (alternativelycalled an update) the final image is formed on the display.

However, in most of the aforementioned methods and controllers, theupdating operation is “atomic” in the sense that once an update isstarted, the memory cannot accept any new image data until the update iscomplete. This causes difficulties when it is desired to use the displayfor applications that accept user input, for example via a keyboard orsimilar data input device, since the controller is not responsive touser input while an update is being effected. For electrophoretic media,in which the transition between the two extreme optical states may takeseveral hundred milliseconds, this unresponsive period may vary fromabout 800 to about 1800 milliseconds, the majority of this period beattributable to the update cycle required by the electro-optic material.Although the duration of the unresponsive period may be reduced byremoving some of the performance artefacts that increase update time,and by improving the speed of response of the electro-optic material, itis unlikely that such techniques alone will reduce the unresponsiveperiod below about 500 milliseconds. This is still longer than isdesirable for interactive applications, such example an electronicdictionary, where the user expects rapid response to user input.Accordingly, there is a need for an image updating method and controllerwith a reduced unresponsive period.

This aspect of the present invention makes use of the known concept ofasynchronous image updating to reduce substantially the duration of theunresponsive period. It is known to use structures already developed forgray scale image displays to reduce the unresponsive period by up to 65percent, as compared with prior art methods and controllers, with onlymodest increases in the complexity and memory requirements of thecontroller.

Finally, this invention relates to a method and apparatus for driving anelectro-optic display in which the data used to define the drive schemeis compressed in a specific manner. The aforementioned MEDEODapplications describe methods and apparatus for driving electro-opticdisplays in which the data defining the drive scheme (or plurality ofdrive schemes) used are stored in one or more look-up tables (“LUT's”).Such LUT's must of course contain data defining the waveform for eachwaveform of the or each drive scheme, and a single waveform willtypically require multiple bytes. As described in the MEDEODapplications, the LUT may have to take account of more than two opticalstates, together with adjustments for such factors as temperature,humidity, operating time of the medium etc. Thus, the amount of memorynecessary for holding the waveform information can be substantial. It isdesirable to reduce the amount of memory allocated to waveforminformation in order to reduce the cost of the display controller. Asimple compression scheme that can be realistically accommodated in adisplay controller or host computer would be helpful in reducing thedisplay controller cost. This invention relates to a simple compressionscheme that appears particularly advantageous for electro-opticdisplays.

SUMMARY OF INVENTION

Accordingly, in one aspect this invention provides a method for drivingan electro-optic display having at least one pixel capable of achievingat least three different gray levels including two extreme opticalstates. The method comprises applying to the pixel a base waveformcomprising at least one reset pulse sufficient to drive the pixel to orclose to one of the extreme optical states followed by at least one setpulse sufficient to drive the pixel to a gray level different from saidone extreme optical state. The base waveform is, however, modified by atleast one of the following:

-   -   (a) insertion of at least one balanced pulse pair into the base        waveform;    -   (b) excision of at least one balanced pulse pair from the base        waveform; and    -   (c) insertion of at least one period of zero voltage into the        base waveform,        where “balanced pulse pair” denotes a sequence of two pulses of        opposite polarity such that the total impulse of the balanced        pulse pair is essentially zero.

Hereinafter, for convenience, this method of the present invention maybe referred to as the “balanced pulse pair slide show” or “BPPSS” methodof the invention. When such a method includes modification of the basewaveform by insertion or excision of at least one balanced pulse pair(“BPP”) the two pulses of the balanced pulse pair may each be ofconstant voltage but of opposite polarity and be equal in length. Whenthe modification of the base waveform includes excision of at least oneBPP, the period in the base waveform occupied by the or each excised BPPmay be replaced by a period of zero voltage; alternatively, otherelements of the base waveform may be shifted in time to occupy theperiod formerly occupied by the or each excised BPP, and a period ofzero voltage may be inserted at a point in time different from thatoccupied by the or each excised BPP.

In a preferred form of the BPPSS method of the present invention, thebase waveform comprises, in succession, a first reset pulse sufficientto drive the pixel to or close to one of its extreme optical states, asecond reset pulse sufficient to drive the pixel to or close to itsother extreme optical state, and the at least one set pulse.

The BPPSS method may be carried out using drive circuitry capable ofvoltage modulation, pulse width modulation or both. However, it is foundespecially useful with tri-level drive schemes in which there is appliedto the pixel at any point in time, a voltage of 0, +V or −V, where V isa predetermined drive voltage.

For reasons explained in detail below, in the BPPSS method, it isdesirable to limit the total number of modifications to the basewaveform (i.e., the total number of inserted or excised balanced pulsepairs and inserted periods of zero voltage). In general, this totalnumber of modifications will not exceed six, desirably will not exceedfour and preferably will not exceed two.

As discussed in the aforementioned MEDEOD applications, and as discussedbelow, it is desirable that the BPPSS method of the present invention beDC balanced, and, as far as possible, it is also desirable that eachindividual waveform of the drive scheme used be DC balanced.

The BPPSS method of the present invention may be used with any of thetypes of electro-optic display discussed above. Thus, for example, thedisplay may comprise a rotating bichromal member or electrochromicmedium. Alternatively, the display may comprise an electrophoreticelectro-optic medium comprising a plurality of electrically chargedparticles in a fluid and capable of moving through the fluid onapplication of an electric field to the fluid. In this type of display,the fluid may be gaseous or liquid. The charged particles and the fluidmay be confined within a plurality of capsules or microcells.

The present invention extends to a display controller, applicationspecific integrated circuit or software code arranged to carry out theBPPSS method of the invention.

In another aspect, this invention provides a method for driving anelectro-optic display having a plurality of pixels each capable ofachieving at least four different gray levels including two extremeoptical states, the method comprising applying to each pixel a waveformcomprising a reset pulse sufficient to drive the pixel to or close toone of its extreme optical states followed by a set pulse sufficient todrive the pixel to a final gray level different from said one extremeoptical state, wherein the reset pulses are chosen such that the imageon the display immediately prior to the set pulses is substantially aninverse monochrome projection of the final image following the setpulses.

Hereinafter, for convenience, this method of the present invention maybe referred to as the “inverse monochrome projection” or “IMP” method ofthe invention. As explained in more detail below, a monochromeprojection of a gray scale image is a projection in which all pixels inthe gray scale image which are in one extreme optical state or in graystates closer to that one extreme optical state than a predeterminedthreshold (for example, white and light gray pixels) are changed to thatextreme optical state (for example, white) or to a state close thereto,while pixels in the opposed extreme optical state or in gray statescloser to this opposed extreme optical state than the threshold (forexample, black and dark gray) are changed to the opposed extreme opticalstate (for example, black) or a state close thereto. An inversemonochrome projection is the reverse of a monochrome projection.

In a preferred form of the IMP method of the present invention, there isapplied to each pixel a waveform comprising a first reset pulsesufficient to drive each pixel to or close to one of its extreme opticalstates, a second reset pulse sufficient to drive each pixel to or closeto the other of its extreme optical states, and the set pulse, and thefirst reset pulses are chosen so that the image on the displayimmediately prior to the second reset pulse is substantially amonochrome projection of the final image following the set pulses.

In the IMP method, the waveform may be modified by:

-   -   (a) insertion of at least one balanced pulse pair into the        waveform;    -   (b) excision of at least one balanced pulse pair from the        waveform; and    -   (c) insertion of at least one period of zero voltage into the        waveform,        where “balanced pulse pair” is as defined above. In such a        modified waveform, the two pulses of the balanced pulse pair may        each be of constant voltage but of opposite polarity and be        equal in length. When the modification of the base waveform        includes excision of at least one BPP, the period in the base        waveform occupied by the or each excised BPP may be replaced by        a period of zero voltage; alternatively, other elements of the        base waveform may be shifted in time to occupy the period        formerly occupied by the or each excised BPP, and a period of        zero voltage may be inserted at a point in time different from        that occupied by the or each excised BPP.

As with the BPPSS method, the IMP method of the present invention may becarried out using drive circuitry capable of voltage modulation, pulsewidth modulation or both. However, the IMP method is found especiallyuseful with tri-level drive schemes in which there is applied to thepixel at any point in time, a voltage of 0, +V or −V, where V is apredetermined drive voltage. Also, as with the BPPSS method, the IMPmethod may be used with any of the types of electro-optic displaydiscussed above. Thus, for example, the display may comprise a rotatingbichromal member or electrochromic medium. Alternatively, the displaymay comprise an electrophoretic electro-optic medium comprising aplurality of electrically charged particles in a fluid and capable ofmoving through the fluid on application of an electric field to thefluid. In this type of display, the fluid may be gaseous or liquid. Thecharged particles and the fluid may be confined within a plurality ofcapsules or microcells.

The present invention extends to a display controller, applicationspecific integrated circuit or software code arranged to carry out theIMP method of the invention.

In another aspect, this invention provides a method for driving anelectro-optic display having at least one pixel capable of achieving atleast two different gray levels, wherein at least two differentwaveforms are used for the same transition between specific gray levelsdepending upon the duration of the dwell time of the pixel in the statefrom which the transition begins, these two waveforms differ from eachother by at least one of the following:

-   -   (a) insertion of at least one balanced pulse pair;    -   (b) excision of at least one balanced pulse pair; and    -   (c) insertion of at least one period of zero voltage,        where “balanced pulse pair” is as defined above.

Hereinafter, for convenience, this method of the present invention maybe referred to as the “dwell time compensation balanced pulse pair” or“DTCBPP” method of the invention. In such a method, the overall drivescheme is very desirably DC balanced, and preferably all waveforms arethemselves DC balanced. When such a method includes modification of thebase waveform by insertion or excision of at least one BPP, the twopulses of the balanced pulse pair may each be of constant voltage but ofopposite polarity and be equal in length. When the modification of thebase waveform includes excision of at least one BPP, the period in thebase waveform occupied by the or each excised BPP may be replaced by aperiod of zero voltage; alternatively, other elements of the basewaveform may be shifted in time to occupy the period formerly occupiedby the or each excised BPP, and a period of zero voltage may be insertedat a point in time different from that occupied by the or each excisedBPP.

As with the BPPSS and IMP methods, the DTCBPP method of the presentinvention may be carried out using drive circuitry capable of voltagemodulation, pulse width modulation or both. However, the DTCBPP methodis found especially useful with tri-level drive schemes in which thereis applied to the pixel at any point in time, a voltage of 0, +V or −V,where V is a predetermined drive voltage. For reasons explained indetail below, in the DTCBPP method, it is desirable to limit the totalnumber of modifications to the base waveform (i.e., the total number ofinserted or excised balanced pulse pairs and inserted periods of zerovoltage). In general, this total number of modifications will not exceedsix, desirably will not exceed four and preferably will not exceed two.

Also, as with the BPPSS and IMP methods, the DTCBPP method may be usedwith any of the types of electro-optic display discussed above. Thus,for example, the display may comprise a rotating bichromal member orelectrochromic medium. Alternatively, the display may comprise anelectrophoretic electro-optic medium comprising a plurality ofelectrically charged particles in a fluid and capable of moving throughthe fluid on application of an electric field to the fluid. In this typeof display, the fluid may be gaseous or liquid. The charged particlesand the fluid may be confined within a plurality of capsules ormicrocells.

The present invention extends to a display controller, applicationspecific integrated circuit or software code arranged to carry out theDTCBPP method of the invention.

In another aspect, this invention provides two related methods forreducing the unresponsive period when an electro-optic display is beingupdated. The first of these methods is for use in driving anelectro-optic display having a plurality of pixels, each of which iscapable of achieving at least two different gray levels, the methodcomprising:

-   -   (a) providing a final data buffer arranged to receive data        defining a desired final state of each pixel of the display;    -   (b) providing an initial data buffer arranged to store data        defining an initial state of each pixel of the display;    -   (c) providing a target data buffer arranged to store data        defining a target state of each pixel of the display;    -   (d) determining when the data in the initial and final data        buffers differ, and when such a difference is found, updating        the values in the target data buffer by (i) when the initial and        final data buffers contain the same value for a specific pixel,        setting the target data buffer to this value; (ii) when the        initial data buffer contains a larger value for a specific pixel        than the final data buffer, setting the target data buffer to        the value of the initial data buffer plus an increment;        and (iii) when the initial data buffer contains a smaller value        for a specific pixel than the final data buffer, setting the        target data buffer to the value of the initial data buffer minus        said increment;    -   (e) updating the image on the display using the data in the        initial data buffer and the target data buffer as the initial        and final states of each pixel respectively;    -   (f) after step (e), copying the data from the target data buffer        into the initial data buffer; and    -   (g) repeating steps (d) to (f) until the initial and final data        buffers contain the same data.

The second of these two methods is for use in driving an electro-opticdisplay having a plurality of pixels, each of which is capable ofachieving at least three different gray levels, the method comprising:

-   -   (a) providing a final data buffer arranged to receive data        defining a desired final state of each pixel of the display;    -   (b) providing an initial data buffer arranged to store data        defining an initial state of each pixel of the display;    -   (c) providing a target data buffer arranged to store data        defining a target state of each pixel of the display;    -   (d) providing a polarity bit array arranged to store a polarity        bit for each pixel of the display;    -   (e) determining when the data in the initial and final data        buffers differ, and when such a difference is found, updating        the values in the polarity bit array and target data buffer        by (i) when the values for a specific pixel in the initial and        final data buffers differ and the value in the initial data        buffer represents an extreme optical state of the pixel, setting        the polarity bit for the pixel to a value representing a        transition towards the opposite extreme optical state; and (ii)        when the values for a specific pixel in the initial and final        data buffers differ, setting the target data buffer to the value        of the initial data buffer plus or minus an increment, depending        upon the relevant value in the polarity bit array;    -   (f) updating the image on the display using the data in the        initial data buffer and the target data buffer as the initial        and final states of each pixel respectively;    -   (g) after step (f), copying the data from the target data buffer        into the initial data buffer; and    -   (h) repeating steps (e) to (g) until the initial and final data        buffers contain the same data.

Hereinafter, for convenience, these two related methods may be referredto as the “target buffer” or “TB” methods of the invention. When it isdesirable to distinguish between the two methods, the former may bereferred to as the “non-polarity target buffer” or “NPTB” method, andthe latter as the “polarity target buffer” or “PTB” method. Thisinvention extends to a display controller, application specificintegrated circuit or software code arranged to carry out the TB methodsof the invention.

Finally, this invention provides a method for reducing the amount ofdata which needs to be stored in order to drive an electro-opticdisplay. Accordingly, this invention provides a method for driving anelectro-optic display having a plurality of pixels, each of which iscapable of achieving at least two different gray levels, the methodcomprising:

-   -   storing a base waveform defining a sequence of voltages to be        applied during a specific transition by a pixel between gray        levels;    -   storing a multiplication factor; and    -   effecting said specific transition by applying to said pixel        said sequence of voltages for periods dependent upon said        multiplication factor.

Hereinafter, for convenience, this method may be referred to as the“waveform compression” or “WC” method of the present invention.

BRIEF DESCRIPTION OF DRAWINGS

As already mentioned, FIG. 1 of the accompanying drawings shows thereflectance of a pixel of an electro-optic display as a function oftime, and illustrates the phenomenon of dwell time dependence.

FIGS. 2A and 2B illustrate waveforms for two different transitions in aprior art three reset pulse slide show drive scheme of a type describedin the aforementioned MEDEOD applications.

FIGS. 2C and 2D illustrate the variations with time of the reflectancesof two pixels of an electro-optic display to which the waveforms ofFIGS. 2A and 2B respectively are applied.

FIGS. 3A and 3B illustrate waveforms for two different transitions in aprior art two reset pulse slide show drive scheme of a type described inthe aforementioned MEDEOD applications.

FIGS. 4A, 4B and 4C illustrate balanced pulse pairs which, in accordancewith the BPPSS method of the present invention, may be used to modifyprior art slide show waveforms such as those shown in FIGS. 2A, 2B, 3Aand 3B.

FIG. 5A illustrates a waveform of a prior art two reset pulse slide showdrive scheme.

FIGS. 5B-5D illustrate BPPSS waveforms of the present invention producedby modifying the waveform of FIG. 5A.

FIG. 6A illustrates the same prior art base waveform as FIG. 5A.

FIGS. 6B-6D illustrate BPPSS waveforms of the present invention producedby excision of balanced pulse pairs from the base waveform of FIG. 6A.

FIG. 7A illustrates a BPPSS waveform of the present invention producedby inserting a balanced pulse pair between two base waveform elements ofa base waveform.

FIG. 7B illustrates a further BPPSS waveform of the present inventionproduced by inserting the same balanced pulse pair as in FIG. 7A withina single base waveform element of the same base waveform as in FIG. 7A.

FIG. 8A illustrates the same prior art base waveform as FIGS. 5A and 6A.

FIGS. 8B-8D illustrate BPPSS waveforms of the present invention producedby insertion of periods of zero voltage at differing locations in thebase waveform of FIG. 8A.

FIGS. 9A and 9B illustrate prior art base waveforms which may bemodified to produce BPPSS waveforms of the present invention.

FIG. 9C illustrates a BPPSS waveform of the present invention producedby insertion of two balanced pulse pairs into the base waveform of FIG.9B.

FIG. 9D illustrates a BPPSS waveform of the present invention producedby insertion of a balanced pulse pair and a period of zero voltage intothe base waveform of FIG. 9B.

FIGS. 10A-10C and 11A-11C illustrate further BPPSS waveforms of thepresent invention produced by modifying the base waveforms of FIGS. 9Aand 9B.

FIG. 12 is a symbolic representation of an inverse monochrome projectionmethod of the present invention.

FIG. 13 shows the manner in which the gray levels of a gray scale imageare mapped to a monochrome projection of the image, as may be effectedin preferred inverse monochrome projection methods of the presentinvention.

FIGS. 14 and 15 show selected waveforms used during a first inversemonochrome projection method of the present invention.

FIG. 16 is a symbolic representation, similar to that of FIG. 12, of afurther inverse monochrome projection method of the present invention.

FIG. 17 illustrates modifications of one of the IMP waveforms shown inFIG. 14 by insertion of balanced pulse pairs into the waveform.

FIG. 18 illustrates modifications of one of the IMP waveforms shown inFIG. 14 by excision of balanced pulse pairs from the waveform.

FIG. 19 illustrates further modifications of one of the IMP waveformsshown in FIG. 17 by variation in the position of insertion of thebalanced pulse pair.

FIG. 20 illustrates further modifications of one of the IMP waveformsshown in FIG. 18 by variation in the position from which the balancedpulse pair is excised.

FIG. 21 illustrates, in a highly schematic manner, the waveforms of afurther IMP drive scheme of the present invention.

FIG. 22 is a graph showing the gray levels produced by the drive schemeshown in FIG. 21.

FIG. 23 illustrates, in the same manner as FIG. 21, a modified form ofthe IMP drive scheme shown in FIG. 21.

FIG. 24 is a graph showing the gray levels produced by the modifieddrive scheme shown in FIG. 23.

FIGS. 25A-25E illustrate a set of dwell time compensated waveforms usedin a first dwell time compensation balanced pulse pair drive scheme ofthe present invention.

FIGS. 26A-26C illustrate a set of dwell time compensated waveforms usedin a second dwell time compensation balanced pulse pair drive scheme ofthe present invention.

DETAILED DESCRIPTION

From the foregoing Summary, it will be seen that the present inventionprovides a number of differing methods for driving electro-opticdisplays, especially bistable electro-optic displays, and apparatus andsoftware code adapted to carry out such methods. The various methods ofthe invention will mainly be described separately below, but it shouldbe understood that a single electro-optic display, or component thereof,may make use of more than one aspect of the present invention. Forexample, a single electro-optic display might make use of the BPPSS, IMPand DTCBPP aspects of the present invention. It should also be notedthat the preferred forms of balanced pulse pairs are common to allaspects of the present invention which make use of such pulse pairs, asare the preferred limitations on the sizes of such pulse pairs and themethods for adjusting the length of waveforms to accommodate insertionor excision of such pairs and/or periods of zero voltage. Finally, itshould be noted that the desirably of DC balanced drive schemes and DCbalanced waveforms, as discussed in the aforementioned MEDEODapplications and below, is also common to all aspects of the presentinvention.

Section A: Balanced Pulse Pair Slide Show Method and Apparatus

As already mentioned, the BPPSS method of the present invention is amethod for driving an electro-optic display having at least one pixelcapable of achieving at least three different gray levels including twoextreme optical states. The method comprises applying to the pixel abase waveform comprising at least one reset pulse sufficient to drivethe pixel to or close to one of the extreme optical states followed byat least one set pulse sufficient to drive the pixel to a gray leveldifferent from said one extreme optical state, the base waveform beingmodified by at least one of the following:

-   -   (a) insertion of at least one balanced pulse pair into the base        waveform;    -   (b) excision of at least one balanced pulse pair from the base        waveform; and    -   (c) insertion of at least one period of zero voltage into the        base waveform.

Also, as already mentioned, the term “balanced pulse pair” denotes asequence of two pulses of opposite polarity such that the total impulseof the balanced pulse pair is essentially zero. In a preferred form ofthe BPPSS method, the two pulses of the balanced pulse pair are each ofconstant voltage but of opposite polarity and are equal in length. Theterm “base waveform element” or “BWE” may be used hereinafter to referto any reset or set pulse of the base waveform. The insertion of thebalanced pulse pair and/or of the zero voltage period (which mayhereinafter be called a “gap”) may be effected either within a singlebase waveform element or between two successive waveform elements. Allthese modifications have the property that they do not affect the netimpulse of the waveform; by net impulse is meant the integral of thewaveform voltage curve integrated over the time duration of thewaveform. Balanced pulse pairs and zero voltage pauses have of coursezero net impulse. Although typically the pulses of a BPP will beinserted adjacent each other, this is not essential and the two pulsesmay be inserted at separate locations.

Where the modification of the base waveform in accordance with the BPPSSmethod includes excision of at least one BPP, the period formerlyoccupied by the or each excised BPP may be left as a period of zerovoltage. Alternatively, this period may be “closed up” by moving some orall of the later waveform elements earlier in time, but in this case itwill normally be necessary to insert a period of zero voltage at somelater stage in the waveform, typically at the end thereof, in order toensure that the overall length of the waveform is maintained, since itis normally necessary to ensure that all pixels of a display are drivenwith waveforms of equal length. Alternatively, of course, the period maybe “closed up” by moving some or all of the earlier waveform elementslater in time, with insertion of a period of zero voltage at someearlier stage of the waveform, typically at the beginning thereof.

As already indicated, the BPPSS waveforms of the present invention aremodifications of base slide show waveforms described in theaforementioned MEDEOD applications. As discussed above, slide showwaveforms comprise one or more reset pulses that cause a pixel to moveto, or at least close to, one extreme optical state (optical rail); ifthe waveform includes two or more reset pulses, each reset pulse afterthe first will cause the pixel to move to the opposed extreme opticalstate, and thus to traverse substantially its entire optical range. (Forexample, if the display uses an electro-optic medium that has a range of(say) 4 to 40 percent reflectance, each reset pulse after the firstmight cause the pixel to traverse from 8 to 35 percent reflectance.) Ifmore than one reset pulse is used, successive reset pulses must ofcourse be of alternating polarity.

A slide show waveform further comprises a set pulse which drives thepixel from the extreme optical state in which it has been left by thelast reset pulse to the desired final gray level of the pixel. Note thatwhen this desired final gray level is one of the extreme optical states,and the last reset pulse leaves the pixel at this desired extremeoptical state, the set pulse may be of zero duration. Similarly, if theinitial state of the pixel before application of the slide show waveformis at one of the extreme optical states, the first reset pulse may be ofzero duration.

Preferred BPPSS waveforms of the present invention will now bedescribed, though by way of illustration only, with reference to theaccompanying drawings.

FIGS. 2A and 2B of the accompanying drawings illustrate the waveformsused for two different transitions in a prior art (base) slide showdrive scheme of a type described In the aforementioned MEDEODapplications. This slide show drive scheme uses three reset pulses foreach transition. FIGS. 2C and 2D show the corresponding variations withrespect to time in optical state (reflectance) of pixels to which thewaveforms of FIGS. 2A and 2B respectively are applied. In accordancewith the convention used in the aforementioned copending applicationSer. Nos. 10/065,795 and 10/879,335, FIGS. 2C and 2D are drawn so thatthe bottom horizontal line represents the black extreme optical state,the top horizontal line represents the white extreme optical state, andintervening levels represent gray states. The beginning and end of thereset and set pulses of the waveforms are indicated in FIGS. 2A and 2Bby broken vertical lines, and the various BWE's (i.e., the reset and setpulses) are shown as consisting of ten or less equal length pulses,although in general the BWE's may be of more arbitrary length and ifcomprised of a series of equal length pulses, more than ten such pulseswould normally be used for a maximum length BWE.

The base waveform (generally designated 100) shown in FIGS. 2A and 2Ceffects a white-to-white transition (i.e., a “transition” in which boththe initial and the final states of the pixel are the white extremeoptical state). The waveform 100 comprises a first negative (i.e.,black-going) reset pulse 102, which drives the pixel to its blackextreme optical state, a second positive (white-going) reset pulse 104,which drives the pixel to its white extreme optical state, a thirdnegative (black-going) reset pulse 106, which drives the pixel to itsblack extreme optical state, and a set pulse 108, which drives the pixelto its white extreme optical state. Each of the four pulses 102, 104,106 and 108 has the maximum ten-unit duration. (To avoid the awkwardnessof continual references to “units of duration”, these units mayhereinafter be referred to as “time units” or “TU's”.)

FIGS. 2B and 2D illustrate a waveform (generally designated 150) for adark gray to light gray transition using the same three reset pulsedrive scheme as in FIGS. 2A and 2C. The waveform 150 comprises a firstreset pulse 152 which, like the first reset pulse 102 of waveform 100,is negative and black-going. However, since the transition for whichwaveform 150 is used begins from a dark gray level, the duration(illustrated as four TU's) of the first reset pulse 152 is shorter thanthat of reset pulse 102, since a shorter first reset pulse is needed tobring the pixel to its black extreme optical state at the end of thefirst reset pulse. For the remaining six TU's of the first reset pulse152, zero voltage is applied to the pixel. (FIGS. 2B and 2D illustratethe first reset pulse 152 with the four TU's of negative voltage at theend of the relevant period, but this is arbitrary and the periods ofnegative and zero voltage may be arranged as desired.)

The second and third reset pulses 104 and 106 of waveform 150 areidentical to the corresponding pulses of waveform 100. The set pulse 158of waveform 150, like the set pulse 108 of waveform 100, is positive andwhite-going. However, since the transition for which waveform 150 isused ends at a light gray level, the duration (illustrated as sevenTU's) of the set pulse 158 is shorter than that of set pulse 108, sincea shorter set pulse is needed to bring the pixel to its final light graylevel. For the remaining three TU's of set pulse 158, zero voltage isapplied to the pixel. (Again, the distribution of periods of positiveand zero voltage within set pulse 158 is arbitrary and the periods maybe arranged as desired.)

From the foregoing, it will be seen that, in the prior art slide showdrive scheme shown in FIGS. 2A-2D, the duration of the first reset pulseand of the set pulse will vary depending upon the initial and finalstates of the pixel respectively, and in certain cases one or both ofthese pulses may be of zero duration. For example, in the drive schemeof FIGS. 2A-2D, a black-to-black transition could have a first resetpulse of zero duration (since the pixel is already at the black extremeoptical state which is reached at the ends of the first reset pulses 102and 152), and a set pulse of zero duration (since at the end of thethird reset pulse 106 the pixel is already at the desired extreme blackoptical state).

In general, it is desirable to keep the overall duration of waveforms asshort as possible so that a display can be rapidly rewritten; forobvious reasons, users prefer displays that display new images quickly.Since each reset pulse occupies a substantial period, it is desirable toreduce the number of reset pulses to the minimum consistent withacceptable gray scale performance by the display, and in general one ortwo reset pulse slide show drive schemes are preferred. FIGS. 3A and 3Bof the accompanying drawings illustrate waveforms for two differenttransitions in a two reset pulse prior art slide show drive scheme ofthe type described in the aforementioned MEDEOD applications.

FIG. 3A illustrates a white to light gray single reset pulse waveform(generally designated 200) comprising a reset pulse 202, which drives apixel from its initial white state to black, and a set pulse 208(identical to pulse 158 in FIG. 2B), which drives the pixel from blackto a light gray. Although waveform 200 uses only a single reset pulse,it will be appreciated that it is actually part of a two reset pulseslide show drive scheme with a first reset pulse of zero duration, asindicated by the period of zero voltage at the left hand side of FIG.3A.

FIG. 3B illustrates a black to light gray two reset pulse waveform(generally designated 250) comprising a first reset pulse 252, whichdrives a pixel from its initial black state to white, a second resetpulse 254, which drives the pixel from white to black, and a set pulse208, identical to the reset pulse in FIG. 3A, which drives the pixelfrom black to light gray.

As already mentioned, the BPPSS waveforms of the present invention arederived from base slide show waveforms such as those illustrated inFIGS. 2A, 2B, 3A and 3B by insertion of at least one balanced pulse pairinto the base waveform, excision of at least one balanced pulse pairfrom the base waveform, or insertion of at least one period of zerovoltage into the base waveform. In the case of excision of a BPP, theresultant gap may be either closed up or left as a period of zerovoltage. Combinations of these modifications may be used.

FIGS. 4A-4C illustrate preferred balanced pulse pairs for use in theBPPSS waveforms of the present invention. The BPP (generally designated300) shown in FIG. 4A comprises a negative pulse 302 of constantvoltage, followed immediately by a positive pulse 304 of the sameduration and voltage as pulse 302 but of opposite polarity. It will beapparent that the BPP 300 applies zero net impulse to a pixel. The BPP(generally designated 310) shown in FIG. 4B is identical to the BPP 300except that the order of the pulses is reversed. The BPP (generallydesignated 320) shown in FIG. 4C is derived from the BPP 310 byintroducing a period 322 of zero voltage between the positive andnegative pulses 304 and 302 respectively.

FIGS. 5A-5D illustrate modifications of a base two reset pulse slideshow waveform by a BPP in accordance with the present invention. FIG. 5Aillustrates the base waveform (generally designated 400) used for awhite to light gray transition. The waveform 400 is generally similar tothe waveform 250 illustrated in FIG. 3B except that the order of the tworeset pulses is reversed. Thus, the waveform 400 comprises a 16-TUnegative black-going first reset pulse 402 (which drives the pixel fromits original white state to its black extreme optical state), a 16-TUpositive white-going second reset pulse 404 (which drives the pixel fromits black extreme optical state to its white extreme optical state) anda 3-TU negative black-going set pulse 408, which drives the pixel fromits white extreme optical state to the desired final light gray state.

FIG. 5B illustrates a BPPSS waveform (generally designated 420) of thepresent invention produced by inserting the BPP of FIG. 4B into thewaveform 400 of FIG. 5A between the second reset pulse 404 and the setpulse 408 thereof. As will be seen from FIG. 5B, the effect of thisinsertion is that the positive pulse 304 of the BPP lengthens the secondreset pulse 404 to 17 TU's, while the negative pulse 302 of the BPPlengthens the set pulse 408 to 4 TU's.

FIG. 5C illustrates a BPPSS waveform (generally designated 440) of thepresent invention produced by inserting the BPP of FIG. 4C into thewaveform 400 of FIG. 5A after the set pulse 408 thereof.

FIG. 5D illustrates a BPPSS waveform (generally designated 460) of thepresent invention produced by further modification of the waveform 420shown in FIG. 5B. The waveform 460 has a second BPP 304′, 302′, insertedbetween its first and second reset pulses 402 and 404 respectively; thissecond BPP is similar to the BPP 304, 302 except that the duration ofboth pulses is doubled.

As already noted and as illustrated in FIG. 5D, the BPPSS waveforms ofthe present invention may include a plurality of BPP's, excisions,pauses and combinations thereof (hereinafter referred to collectively as“additional waveform elements” or “AWE's”). However, in general it ispreferred to use the minimum number of AWE's consistent with the desireddegree of precision in control of the final gray level produced by thewaveform. BPP's and pauses both lengthen the waveform, and incorporationof several such BPP's and/or pauses may require an undesirablelengthening of the period required for rewriting of the display. Forexample, although the waveform 460 of FIG. 5D uses only a short 3-TU setpulse 408, the waveform 460 occupies the full period for updating of thedisplay (the period between the broken vertical lines in FIG. 5D), andintroduction of any further BPP's or pauses would require extending thisperiod. Thus, the total length of a modified waveform of the presentinvention desirably does not exceed that of the corresponding basewaveform in which the duration of the set pulse is sufficient to drivethe pixel from one extreme optical state to the other. In many cases(depending of course upon the exact electro-optic medium used in thedisplay and other characteristics of the drive electronics), it has beenfound that good control of gray levels can be achieved with waveformscomprising not more than two AWE's; in other cases, not more than four,or less commonly not more than six, AWE's may be required, but anyfurther increase in AWE's is generally undesirable.

FIGS. 6A-6D illustrate modifications of a base two reset pulse waveformby excision of a BPP in accordance with the present invention. Forpurposes of comparison, FIG. 6A illustrates the same waveform 400 asFIG. 5A. Note that the waveform 400 is regarded as terminating 7 TU'safter the end of set pulse 408 since FIG. 6A assumes that, as in FIGS.2A, 2B, 3A and 3B, 10 TU's of the applied voltage is required to drivethe pixel completely between its extreme optical states, so that inother waveforms of the same drive scheme, it will be necessary tolengthen set pulse 408 up to a maximum of 10 TU's. FIG. 6B illustrates amodified BPPSS waveform (generally designated 520) of the presentinvention produced by excising from the waveform 400 a BPP comprisingthe last two TU's of the first reset pulse 402 and the first two TU's ofthe second reset pulse 404, leaving a modified 14-TU first reset pulse402′ and a modified 14-TU second reset pulse 404′, separated by a 4-TUpause 522, during which zero voltage is applied to the pixel.

FIG. 6C illustrates a BPPSS waveform (generally designated 540) of thepresent invention produced by an alternative modification of thewaveform 400 of FIG. 6A. The waveform 540 is produced by excising fromthe waveform 400 a BPP comprising the last TU of the second reset pulse404 and the first TU of the set pulse 408, and “closing up” the periodoriginally occupied by the excised BPP by moving the first and secondreset pulses later in time by 2 TU's. Thus, the waveform 540 comprises a2-TU pause 544, a 16-TU first reset pulse 402, a 15-TU second resetpulse 404″ and a 2-TU set pulse 408; note that the set pulse 408′terminates at exactly the same time as the set pulse 408 of the basewaveform 400, 7 TU's before the end of the waveform.

FIG. 6D illustrates a BPPSS waveform (generally designated 560) of thepresent invention produced by a further modification of the waveform 400of FIG. 6A. The waveform 560 is produced by excising from the waveform400 a BPP comprising the last 2 TU's of the first reset pulse 402 andthe first 2 TU's of the second reset pulse 404, and “closing up” theperiod originally occupied by the excised BPP by moving the second resetpulse and the set pulse earlier in time by 4 TU's. Thus, the waveform560 comprises a 14-TU first reset pulse 402′ (identical to that in FIG.5B), a 14-TU second reset pulse 404′ (identical except for timing tothat in FIG. 5B) and a 3-TU set pulse 408. Note that because of theshift of the second reset pulse 404′ and the set pulse 408, the finalperiod 562 of zero voltage following the set pulse 408 is extended from7 to 11 TU's.

The preferred BPPSS waveform modifications discussed so far haveinvolved insertion or excision of BPP's between successive base waveformelements or at the end of the base waveform. However, the BPPSS aspectof the present invention is not limited to such modifications, butextends to modifications in which a BPP is inserted within a single BWE,as will now be illustrated with reference to FIGS. 7A and 7B. FIG. 7Aillustrates a BPPSS waveform 620 of the present invention produced bymodifying base waveform 400 (FIG. 5A or 6A) by insertion between thefirst reset pulse 402 and the second reset pulse 404 of a BPP 302′, 304′similar to that shown in FIG. 5D except that the order of the positiveand negative pulses is reversed. FIG. 7B illustrates a further BPPSSwaveform 640 of the present invention also produced by modifying basewaveform 400 by insertion of a BPP 302′, 304′, but in waveform 640 theBPP 302′, 304′ is inserted at the mid-point of the second reset pulse404, thus splitting this pulse into two separate sections 404A and 404B.Thus, waveform 640 comprises, in succession, a 16-TU first reset pulse402 (identical to that of waveform 400), the 8-TU pulse 404A, the firstsection of the second reset pulse, the BPP 302′, 304′, the 8-TU pulse404B, the second section of the second reset pulse, and a 3-TU resetpulse 408 (identical to that of waveform 400).

As already mentioned, the BPPSS aspect of the present invention includesnot only the insertion or excision of BPP's from base waveforms but alsothe insertion of pauses (periods of zero voltage) into base waveforms,and such insertion of pauses will now be illustrated with reference toFIGS. 8A-8D. For purposes of comparison, FIG. 8A illustrates the samebase waveform 400 as FIGS. 5A and 6A. FIG. 8B illustrates a modifiedBPPSS waveform (generally designated 720) of the present inventionproduced by introducing into the base waveform 400 between the secondreset pulse 404 and the set pulse 408 thereof a 2-TU pause 722. Itshould be noted that insertion of the pause 722 necessarily reduces thelength of the period of zero voltage following set pulse 408 from 7 to 5TU's. FIG. 8C illustrates another BPPSS waveform (generally designated740) of the present invention generally similar to waveform 720 exceptthat the 2-TU pause is inserted after the first 12 TU's of the secondreset pulse 404, thus splitting this second reset pulse into a firstsection 404C and a second section 404D. Thus, waveform 740 comprises, insuccession, a 16-TU first reset pulse 402 (identical to that of waveform400), the 12-TU pulse 404C, the first section of the second reset pulse,the 2-TU pause 722′, the 4-TU pulse 404D, the second section of thesecond reset pulse, and the 3-TU reset pulse 408 (identical to that ofwaveform 400).

FIG. 8D illustrates a BPPSS waveform (generally designated 760) of thepresent invention which is again produced by insertion of a 2-TU pauseinto the base waveform 400. However, in the waveform 760, the pause 722″is inserted prior to the first reset pulse 402. Thus, the waveform 760comprises, in succession, the pause 722″, the first reset pulse 402, thesecond reset pulse 404 and the set pulse 408, the last three elementsall being identical to the corresponding elements of the base waveform400.

As already indicated, the BPPSS waveforms provided by the presentinvention are useful for improving the gray level performance ofelectro-optic displays, especially bistable electro-optic displays. TheBPPSS waveforms of the present invention can achieve such improved graylevel performance while still preserving long term DC balancing of thedisplay. (For reasons discussed in detail in the aforementioned MEDEODapplications, it is important that drive schemes used to drive at leastsome electro-optic displays be DC balanced, in the sense that theintegral of the applied voltage with respect to time for an given pixelbe bounded regardless of the series of optical states through which thatpixel is driven.) It has been found that the final gray level of a pixelcan be adjusted by insertion or excision of BPP's and/or insertion ofpauses in accordance with the BPPSS aspect of the present invention. Ithas also been found that the final gray level of a pixel is affected bythe position(s) at which the insertion or excision of BPP's and/orinsertion of pauses is effected. While in general good control of finalgray levels can be effected by inserting BPP's between adjacent BWE's,BPP's may be inserted within a single BWE, as illustrated in FIG. 7B, tochange the degree of “tunability” of the final gray level; for example,if a BPP added between two reset pulses does not provide sufficientlyfine tunability of the final gray level, moving the BPP to a point inthe middle of a BWE can give finer adjustment of the final gray level.

For example, the waveform 420 of FIG. 5B would normally produce a graylevel slightly darker than the gray level produced by the correspondingbase waveform 400 of FIG. 5A because the pulse 304 of the BPP 304, 302will have little or no effect on the gray level of the pixel, since thisgray level will already be at the white extreme optical state at the endof the second reset pulse 404, whereas the pulse 302, by effectivelylengthening the set pulse 408, will cause the final gray level to besomewhat further from the white extreme optical state (i.e., slightlydarker in color). In contrast, the waveform 540 shown in FIG. 6C wouldnormally produce a gray level slightly lighter than the gray levelproduced by the corresponding base waveform 400 of FIG. 6A. Since FIGS.5A, 6A and 6C are based upon the assumption that the pixel can beshifted between its extreme optical states by application of theillustrated voltage for 10 TU's (as mentioned above), the 16-TU secondreset pulse 404 of base waveform 400 effects substantial “over-driving”of the pixel into the white optical rail (white extreme optical state),i.e., the second reset pulse 404 continues for a substantial periodafter the pixel has already reached its extreme white optical state.Hence, shortening the 16-TU second reset pulse 404 by 1 TU to producethe 15-TU second reset pulse 404″ of waveform 540 will have little or noeffect on the gray level at the end of the second reset pulse 404″. Incontrast, the shortening of the 3-TU set pulse 408 of waveform 400 by 1TU to produce the 2-TU set pulse 408′ of waveform 540 will significantlyreduce the extent to which the white extreme optical state present atthe end of the second reset pulse 404″ is driven towards black, so thatthe final gray level at the end of waveform 540 will be significantlydarker than at the end of base waveform 400.

As already indicated, it has also been found that pauses (periods ofzero voltage) can be used to adjust the final gray level. For example,adding a pause between the last reset pulse and the set pulse affectsthe final gray level. Moving the pause to an earlier point in the lastreset pulse also induces slight changes in the final gray level. Thus,pause location can be used to adjust the final gray level produced by aBPPSS waveform. In general pauses can be added at any point in awaveform. Furthermore, it may be advantageous to shift all the BWE's ofa waveform earlier or later in time within an allotted update timeinterval for full rewriting of a display, thereby shifting the relativetemporal positioning of the various transitions taking place within theoverall transition from an initial state to a final state. Such temporalshifting may be advantageous for several reasons, for example to reduceundesirable transient behavior of the display during transitions. or tolead to a more pleasing final image, for example by reducing variationsbetween pixels which are intended to be at the same gray level.

Further preferred BPPSS waveforms and drive schemes of the presentinvention will now be described with reference to FIGS. 9A-9D, 10A-10Cand 11A-11C of the accompanying drawings. FIGS. 9A and 9B illustrate twobase waveforms of a prior art two reset pulse slide show drive scheme,in which each of the first and second reset pulses and the set pulse mayoccupy a maximum of 12 TU's. FIG. 9A illustrates a waveform 800 foreffecting a white-to-black transition, and comprising a 12-TUblack-going first reset pulse 802, a 12-TU second white-going resetpulse 804, and a 12-TU black-going set pulse 808. As discussed abovewith reference to FIGS. 2A and 2B, if the initial and final states of apixel are intermediate gray levels lying between the black and whiteextreme optical states of the pixel, the first reset and set pulses needto be adjusted in length, and FIG. 9B shows a base waveform 810comprising a 7-TU first reset pulse 812, a 12-TU second reset pulse 804(identical to the corresponding pulse of waveform 800) and a 6-TU setpulse 818. To “pad” the waveform 810 to the same overall 36-TU length asthe waveform 800, a 5-TU period 822 of zero voltage precedes the firstreset pulse 812 and a 6-TU period 824 of zero voltage follows the setpulse 824.

FIG. 9C shows a BPPSS waveform (generally designated 840) of the presentinvention produced by modification of the waveform 810 shown in FIG. 9B.Specifically, waveform 840 is derived from waveform 810 by inserting afirst BPP, comprising a positive 1-TU pulse 842 and a similar negativepulse 844, immediately before the first reset pulse 812 and a second,similar BPP 846, 848 immediately after the set pulse 818. The pulses812, 804 and 818 are unaltered, but to accommodate the BPP's whilemaintaining the overall length of the waveform 840, the initial period822′ of zero voltage is reduced to 3 TU's, and the final period 824′ ofzero voltage is reduced to 4 TU's.

The use of two BPP's in the manner illustrated in FIG. 9C can, in atleast some cases, enable more precise control of final gray level thancan be achieved with a single BPP. It has been found that a BPP disposedafter the set pulse (such as the BPP 846, 848 in waveform 840) can causea significant change in the final gray level, and if the driver usedonly allows relatively coarse adjustment of the duration of each half ofthe BPP (if, for example, this duration can only be adjusted inincrements of 1 TU in FIG. 9C), the difference between the gray levelsavailable by changing the duration of each half of the BPP by theminimum increment may be unacceptably large. A BPP (such as the BPP 842,844 in waveform 840) inserted at a much earlier point in the waveformhas a much smaller effect on final gray level than a BPP inserted afterthe set pulse, and hence allows for finer variation of final gray level.Thus, the waveform 840 permits adjustment of final gray level over aconsiderable range by controlling the duration of the BPP 846, 848 toeffect coarse adjustment of the final gray level and controlling theduration of the BPP 842, 844 to effect fine adjustment of this graylevel.

FIG. 9D illustrates a BPPSS waveform (generally designated 860) of thepresent invention produced by an alternative modification of waveform810. Like waveform 840, waveform 860 comprises a BPP 846, 848 followingthe set pulse 818. However, the waveform 860 does not include a secondBPP earlier in the waveform, but instead includes a 4-TU pause 850between the second reset pulse 804 and the set pulse 818. The effect ofa pause tends to be smaller than a BPP of the same length at the samepoint in the waveform, and the pause 850 acts in a similar manner to theBPP 842, 844 of waveform with variation of the length of the pause 850serving to effect fine adjustment of the final gray level. Note than inwaveform 860 the final period 824′ of zero voltage is of the same 4-TUlength as in waveform 840, but the duration of the initial period 822″of zero voltage is reduced to 1 TU to accommodate the 4-TU pause 850while still maintaining the overall 36-TU length of the waveform.

FIGS. 10A-10C show three further BPPSS waveforms of the inventionproduced by various modifications of the waveform 810 of FIG. 9B. Thewaveform (generally designated 920) of FIG. 10A is formed by adding aBPP 846′, 848′ after the set pulse 818 of waveform 810 (FIG. 9B), eachpulse 846′ and 848′ of the BPP being 2 TU's in length. The final period824″ of zero voltage is reduced to 2 TU's to accommodate the 4-TU lengthof the BPP.

As discussed above with reference to FIG. 9C, varying the length of aBPP following the set pulse may not provide sufficiently fine adjustmentof the final gray level, and FIG. 10B illustrates a waveform (generallydesignated 940) produced by further modifying waveform 920 to overcomethis fine tuning problem. The waveform 940 incorporates a second BPP842′, 844′ between the second reset pulse 804 and the set pulse 818. Theeffect on the final gray level of varying the length of BPP 842′, 844′is less than a corresponding variation of the length of BPP 846′, 848′,and hence BPP 842′, 844′ can be used for fine adjustment of the finalgray level.

Although the effect of varying the length of BPP 842′, 844′ is less thana corresponding variation of the length of BPP 846′, 848′, it is stillgreater than the effect of varying the length of a BPP inserted stillearlier in the waveform, for example BPP 842, 844 in FIG. 9C. If BPP842′, 844′ in waveform 940 fails to provide sufficiently fine adjustmentof the final gray level, the second BPP may be inserted earlier in thewaveform; in general, the earlier in the waveform a BPP is inserted, thesmaller the variation in final gray level produced by an given change inthe length of the BPP. For example, FIG. 10C illustrates a BPPSSwaveform (generally designated 960) of the present invention which issimilar to waveform 940 except that the BPP 842′, 844′ is replaced by aBPP 962, 964 disposed between the first reset pulse 812 and the secondreset pulse 804. (The BPP 962, 964 is of opposite polarity to BPP 842′,844′ in the sense that the negative pulse 962 precedes the positivepulse 964; BPP's of either polarity may be used in any location withinthe waveform, although of course the polarity of a BPP does alter itseffect upon the final gray level.)

Finally, FIGS. 11A-11C illustrate modification of base waveforms byintroducing both BPP's and pauses therein. FIG. 11A illustrates awaveform (generally designated 1020) produced by modifying base waveform810 by inserting a BPP 842′, 844′ between the second reset pulse 804 andthe set pulse 818, and with a corresponding reduction of the length ofthe final period 824′ of zero voltage to 4 TU's. For reasons discussedabove, variation of the length of BPP 842′, 844′ may not providesufficiently fine adjustment of the final gray level, and FIG. 11B showsa BPPSS waveform (generally designated 1040) produced by furthermodification of waveform 1020, specifically by introduction of a 2-TUpause 1042 within the second reset pulse, thus dividing this pulse intoa first section 804A and a second section 804B. To accommodate the pause1042, the length of the initial period 822′ of zero voltage is reducedto 3 TU's; the length of the final period 824′ of zero voltage remainsat 5 TU's.

The pause 1042 is used for fine adjustment of the final gray level. Suchfine adjustment may be effected by varying the duration of the pause1042 and/or its position within the second reset pulse 804A, 804B; aswith a BPP, the effect of a pause on the final gray level varies notonly with its length but also with its position within the waveform. TheBPPSS aspect of the present invention is of course not confined to theuse of a single pause; for example, the pause 1042 could be replaced bytwo separate pauses each of 1 TU duration, so that the second resetpulse would be split into three sections rather than two.

As already mentioned, when a waveform does not occupy the full periodavailable for updating the display (as for example with the waveform 810of FIG. 9B, which occupies only 25 TU's, whereas a period of at least 36TU's is needed for updating the display to accommodate the longerwaveform 800 of the same drive scheme), it may be advantageous to shiftthe overall waveform within the updating period, for example to reducetransient visual effects during updating. FIG. 11C illustrates awaveform (generally designated 1060) which is produced by shifting theentire waveform 1040 of FIG. 11B earlier in time by 2 TU's (in effectinserting a 2-TU gap immediately after the set pulse 818, as indicatedin FIG. 11C), thus reducing the initial period 822″ of zero voltage toonly 1 TU, and increasing the length of the final period 824A of zerovoltage to 6 TU's.

Section B: Inverse Monochrome Projection Method and Apparatus

As already mentioned, a second aspect of the present invention providesa method for driving an electro-optic display having a plurality ofpixels each capable of achieving at least four different gray levelsincluding two extreme optical states. The method comprises applying toeach pixel a waveform comprising a reset pulse sufficient to drive thepixel to or close to one of its extreme optical states followed by a setpulse sufficient to drive the pixel to a final gray level different fromsaid one extreme optical state. The reset pulses are chosen such thatthe image on the display immediately prior to the set pulses issubstantially an inverse monochrome projection of the final imagefollowing the set pulses. Such a process is referred to herein as an“inverse monochrome projection” or “IMP” method.

Using the “goal state” nomenclature as used in Scheme 1 above, an IMPmethod may be defined as one in which the final goal state isapproximately an inverse monochrome projection of the desired finalstate (R₁) of the display. In a preferred form of the IMP method, thegoal state immediately prior to the final goal state (goal_(n-1) in thenomenclature of Scheme 1) is approximately a monochrome projection ofthe desired final state (R₁) of the display. Such a preferred IMPprocess may be represented symbolically as in Scheme 2 shown in FIG. 12,in which R_(1,m) represents the monochrome projection of R₁, and theover-lining indicates image reversal.

A monochrome projection of an optical state is a mapping of all possiblegray levels in the image to one of the two extreme optical states ofeach pixel or (for reasons explained below) a state close to one of theextreme optical states. For present purposes, the gray levels may bedenoted 1, 2, 3, . . . , N, where N is the number of gray levels, andthe gray level with the smallest reflectance (typically, black) isdenoted 1, the gray level with the next smallest reflectance 2, and soon up to the gray level (typically, white) with the largest reflectancebeing denoted N. A monochrome projection of a gray scale image is onewhereby the gray levels equal to or below a threshold are mapped to graylevel 1, or a state close thereto and the gray levels greater than thethreshold are mapped to gray level N, or a state close thereto. Thethreshold is most desirably N/2, but in practice can usefully be setanywhere within the middle half of the range from 1 to N, that is, thethreshold is at least N/4 and at most 3N/4.

An example of a monochrome projection is shown in FIG. 13. In thisexample, the gray scale image (illustrated in a symbolic manner on theleft hand side of FIG. 13) contains eight gray levels, denoted 1 to 8.Gray levels 1 to 3 are mapped, in the monochrome projection shownsymbolically on the right hand side of the Figure, to gray level 1, asindicated by the connecting lines, while gray levels 4 to 8 are mappedto gray level 8. An inverse monochrome projection is of course producedsimply by reversing the two states used in a monochrome projection

The preceding references to the IMP method producing “substantially” aninverse monochrome projection, and such a projection involving opticalstates “close to” one of the extreme optical states, requireexplanation. In principle, monochrome projections and inverse monochromeprojections require projection to one of the extreme optical states.However, in practice drive schemes and waveforms for drivingelectro-optic displays are defined in terms of the voltage pulses orother waveform elements applied to the individual pixels of a display,not in terms of the exact optical states which result from applicationof the defined voltage pulses or other waveform elements (although thetwo are closely related). As discussed in detail in the aforementionedMEDEOD applications, the response of at least some bistableelectro-optic media to a given waveform or waveform element depends notonly upon the initial optical state of the pixel and the exact waveformor waveform element, but also upon factors such as certain prior opticalstates of the pixel, and how long the pixel has remained in the sameoptical state before the waveform or waveform element is applied (theaforementioned dwell time dependency problem). Since slide showwaveforms typically do not allow for all such relevant factors, theactual optical states achieved by various pixels in a monochromeprojection or inverse monochrome projection may differ slightly from theextreme optical states theoretically achieved in such projections.

This deviation of the actual optical states of pixels from extremeoptical states may be illustrated with reference to FIGS. 14 and 15,which show waveforms used for certain selected transitions in an tworeset pulse slide show IMP method of the present invention using a fourgray level electro-optic medium which can be driven from black (graylevel 1) to white (gray level 4) using a +15 V 200 msec pulse, and fromwhite to black using a −15V 200 msec pulse. The first waveform(generally designated 1420) shown in FIG. 14 is for the black (graylevel 1) to white (gray level 4) transition, and comprises a first resetpulse 1422, which drives the pixel from black to white, a second resetpulse 1424, which drives the pixel from white to black, and a set pulse1426, which drives the pixel from black to white. FIG. 14 also shows awaveform 1440 for the gray level 2 (dark gray) to gray level 4 (white)transition; this waveform 1440 has a first reset pulse 1428 which isonly 140 msec in length, rather than 200 msec as in the case of resetpulse 1422 of waveform 1420. The second reset pulse 1424 and the setpulse 1426 of waveform 1440 are identical to those of waveform 1420.Finally, FIG. 14 also shows a waveform 1460 for the gray level 4 (white)to gray level 4 transition; in this case, the first reset pulse is ofzero duration (i.e., there is simply a 200 msec period of zero voltageat the beginning of the waveform) but the second reset pulse 1424 andthe set pulse 1426 of waveform 1460 are identical to those of waveform1420.

FIG. 15 shows additional waveforms from the same drive scheme as in FIG.14. The first waveform (generally designated 1480) shown in FIG. 15 isfor the gray level 1 (black) to gray level 1 transitions and, isessentially the inverse of waveform 1460 shown in FIG. 14. Waveform 1480has a first reset pulse is of zero duration (i.e., there is simply a 200msec period of zero voltage at the beginning of the waveform), a secondreset pulse 1482, which drives the pixel from black to white, and a setpulse 1484, which drives the pixel from white to black. FIG. 15 alsoillustrates a waveform 1500 used for the gray level 1 (black) to graylevel 3 (light gray) transition. This waveform 1500 has a first resetpulse 1422 which is identical to that of waveform 1420 shown in FIG. 14and drives the pixel from black to white. Waveform 1500 also has asecond reset pulse 1502, which drives the pixel from white to black, anda 130 msec set pulse 1504, which drives the pixel from black to graylevel 3 (light gray). Finally, for completeness, FIG. 15 repeats theblack to white (gray level 1 to gray level 4) waveform from FIG. 14.

From FIGS. 14 and 15, it will be seen that the drive scheme illustratedis an IMP drive scheme in that, as indicated by the over-lined R1,mimmediately before the set pulses in the various waveforms, the image onthe display immediately before the set pulses is an inverse monochromeprojection of the final image after the set pulses; more specifically,in all transitions which end at gray level 3 or 4, the pixel is blackimmediately before the set pulse, whereas for all transitions which endat gray level 1 or 2, the pixel is white immediately before the setpulses. Furthermore, in accordance with the preferred variant of the IMPmethod, as indicated by the R1,m immediately before the second resetpulses in the various waveforms, the image on the display immediatelybefore the second reset pulses is an monochrome projection of the finalimage after the set pulses; more specifically, in all transitions whichend at gray level 3 or 4, the pixel is white immediately before thesecond reset pulse, whereas for all transitions which end at gray level1 or 2, the pixel is black immediately before the second reset pulse.

However, it can be deduced from FIGS. 14 and 15 that the reflectance ofa given gray level achieved at various points in the various waveformsis not necessarily precisely the same, although the differences betweenpixels supposedly at the same gray level will be small relative to thetotal dynamic range (the difference between the reflectances of the twoextreme optical states) of the display. For example, immediately beforethe second reset pulse, pixels undergoing waveforms 1420 and 1460 inFIG. 14 should both be at gray level 4 (white). However, a pixelundergoing waveform 1420 will at this point have just completed ablack-to-white transition, whereas a pixel undergoing the waveform 1460may have been in the white state for some time and (as discussed in someof the aforementioned MEDEOD applications) there is a tendency foroptical states of bistable electro-optic media to “drift” (i.e., changegradually with time) while they are not being driven. Hence, the actualwhite state of a pixel undergoing the waveform 1460 may differ slightlythat of a freshly re-written pixel undergoing the waveform 1420.Modifications to an IMP drive scheme, such as those discussed below, maymodify the reflectances achieved at the various goal states and otherpoints in waveforms, and thus the reflectance of the various goal andother states can deviate considerably from the reflectance at the goalstate one would have achieved without such modification.

Although the IMP drive scheme illustrated in FIGS. 14 and 15 uses onlytwo reset pulses and thus two goal states, the IMP aspect of the presentinvention is of course not confined to a specific number of reset pulsesand goal states; for example, FIG. 16 illustrates symbolically, in thesame way as FIG. 12, an IMP drive scheme which includes intermediateblack (B) and white (W) states prior to the monochrome projection andinverse monochrome projection goal states.

It should be noted that not all pixels of a display necessarily reach agiven goal state (for example, the inverse monochrome projections goalstate) at the same point in time during rewriting of a display from aninitial image to a desired final image. The time point in a transitionat which the goal states are reached are functions of the initial anddesired final gray levels, R2 and R1, respectively. Ideally (and asnormally illustrated herein), the time points for R2 and R1 match, withthe entire display being driven through various goal states, and thesegoal states being reached simultaneously by all pixels. However, it isoften desirable to shift the relative timing of the various waveforms ofa drive scheme. Time shifting of the waveforms may be done for aestheticreasons, for example, to improve the appearance of the transition or theappearance of the resulting image. Also, modifications such as thosediscussed below may shift the relative time positions of the goalstates, so that for various combinations of R1 and R2, the goal statesare reached at different times during a transition.

It is possible to give an alternative definition of an IMP drive schemewithout explicit reference to inverse monochrome projections. An IMPdrive scheme is one in which the various gray levels of a display can bedivided by a threshold such that one extreme optical state and at leastone non-extreme optical state lie on each side of the threshold, and theset pulses of a slide show drive scheme are defined such that each setpulse effects a transition across the threshold. As this definitionmakes clear, in an IMP drive scheme, the final set pulse of eachwaveform drives the pixel to the desired final gray level from theextreme optical state further from this desired final gray level, where“further” is used to indicate “on the opposed side of the threshold”rather than simply counting the number of gray levels difference betweenthe desired final gray level and the two extreme optical states.

It has been found that IMP drive schemes allow precise control of finalgray levels and offer wide temperature performance ranges. It isbelieved (although the invention is in no way limited by this belief)that these advantages are linked to the relatively long set pulses usedto drive from the “further” extreme optical state to the final graylevel, and the consequent relatively constant power drain on the driveelectronics during display updating.

The basic IMP drive schemes described above can usefully be modified inseveral different ways to make small adjustments in the final graylevels achieved, to change the appearance of the display duringtransitions and to achieve desirable image quality.

The first type of modification of IMP drive schemes is insertion orexcision of balanced pulse pairs, and/or insertion of period of zerovoltage into the waveforms, in a manner similar to that effected inBPPSS drive schemes, as discussed in Section A above. The balanced pulsepairs used may, for example, have any of the forms shown in FIGS. 4A-4C.The modifications of a basic IMP waveform to insert or excise BPP's orinsert periods of zero voltage (pauses) may be effected in any of theways previously described. A BPP may be inserted between two consecutivebase waveform elements or within a single base waveform element. In manycases, this has the effect of increasing the pulse length both to andaway from a particular goal state. An excised BPP may be replaced by aperiod of zero voltage, or other base waveform elements may be shiftedin time to “close up” the period previously occupied by the excised BPP,and periods of zero voltage may be inserted at other points in thewaveform. As in BPPSS drive schemes, the final gray level achieved issensitive not only to the presence of BPP's and pauses in the waveformbut also to their positioning within the waveform, with the general rulebeing that the earlier in a waveform a BPP is inserted or excised or apause is inserted, the smaller the effect of the change on the finalgray level.

It is important to realize that such waveform modifications will affectnot only the reflectance not only of the final optical state (i.e., thefinal gray level), but also the intermediate goal states. While the goalstates of a basic IMP waveform are generally near one of the extremeoptical states (optical rails), and, by definition, are near the opticalrails for the last goal state, or last two goal states in the preferredform of an IMP drive scheme, the modifications described above can shiftthe reflectance at a goal state away from an optical rail. It is thechange in the degree of drive toward an optical rail that gives smalladjustments in the final optical state (gray level).

It has been found desirable to keep the impulses of each of the voltagepulses comprising a BPP relatively small. The magnitude of a BPP may bedefined by a parameter d, the absolute value of which describes thelength of each of the two voltage pulses of a BPP, and the sign of whichdenotes the sign of the second of the two pulses. For example, the BPP'sshown in FIGS. 4A and 4B can be assigned d values +1 and −1,respectively (while the BPP of FIG. 4C is then, in a consistent scheme,assigned a d value of −1 with a gap modification inserted between thetwo pulses). In a preferred embodiment of the IMP drive scheme, allBPP's used have d values whose magnitudes are less than PL, andpreferably less than PL/2, where PL (in the same units used to measurethe BPP's) is defined as the length of the voltage pulse required todrive a pixel from one extreme optical state to the other, or theaverage value of this voltage pulse where the lengths for transitions inthe two directions are not the same, at a drive voltage characteristicof the drive scheme. In the example just given, d is expressed in unitsof display scan frames, and the BPP's of FIGS. 4A and 4B have voltagepulses each one scan frame in length. In this case, PL would also bedefined in scan frames. All quantities could of course alternatively beexpressed in a time unit, such as seconds or milliseconds.

FIG. 17 of the accompanying drawings illustrates three waveformsproduced by modifying the IMP waveform 1440 shown in FIG. 14 byinsertion of a BPP. The first waveform (generally designated 1700) shownin FIG. 17 is identical to waveform 1440 except that a BPP 1702,comprising a −15V 10 msec pulse followed by a +15V 10 msec pulse isinserted at the end of the waveform. The second waveform (generallydesignated 1720) shown in FIG. 17 inserts a BPP 1722, identical to theBPP 1702, but inserted between the second reset pulse and the set pulseof the waveform; to accommodate BPP 1722, the two reset pulses areshifted earlier in time by 20 msec, with a corresponding reduction inthe period of zero voltage at the beginning of the waveform. The thirdwaveform (generally designated 1740) shown in FIG. 17 has a BPP 1742inserted between the first and second reset pulses of the waveform; BPP1742 has the order of its pulses reversed as compared with BPP's 1702and 1722 and each pulse is 20 msec in length. To accommodate BPP 1742,the first reset pulse is shifted earlier in time by 40 msec, with acorresponding reduction in the period of zero voltage at the beginningof the waveform.

FIG. 18 of the accompanying drawings illustrates three waveformsproduced by modifying the IMP waveform 1440 shown in FIG. 14 by excisionof a BPP therefrom. The first waveform (generally designated 1760) shownin FIG. 18 is produced by excising from waveform 1440 a BPP 1762comprising the last 10 msec scan frame of the second reset pulse and thefirst scan frame of the set pulse, with no change in the remainingwaveform elements. The second waveform (generally designated 1780) shownin FIG. 18 is similarly produced by excising from waveform 1440 a BPP1782 comprising the last two scan frames of the first reset pulse andthe first two scan frames of the second reset pulse, with no change inthe remaining waveform elements, thus leaving a 40 msec period of zerovoltage at the point occupied by the excised BPP. Finally, the thirdwaveform (generally designated 1800) shown in FIG. 18 is produced byexcising from waveform 1440 a BPP comprising the last scan frame of thefirst reset pulse and the first scan frame of the second reset pulse,and closing up the resultant gap by moving the remaining scan frames ofthe first reset pulse 20 msec later in time, with a correspondingincrease in the period of zero voltage at the beginning of the waveform.

FIG. 19 of the accompanying drawings illustrates possible furthermodification of the waveform 1720 shown in FIG. 17. The upper part ofFIG. 19 repeats the basic waveform 1720, including BPP 1722, from FIG.17. FIG. 19 also illustrates a modified waveform (generally designated1920) which comprises a BPP 1922 similar to BPP 1722 but inserted 40msec earlier in time, before the last four scan frames of the secondreset pulse. FIG. 19 also illustrates a second modified waveform(generally designated 1940) which comprises a BPP 1942 similar to BPP1722 but inserted 130 msec earlier in time, before the last thirteenscan frames of the second reset pulse. As already noted, the final graylevel achieved by waveforms such as those shown in FIG. 19 is a functionof the position of insertion of the balanced pulse pair, somodifications such as those shown in FIG. 19 can be used for fine tuningof the final gray level.

FIG. 20 of the accompanying drawings illustrates modified IMP waveformsproduced by inserting periods of zero voltage (pauses) into the basicIMP waveform 1440 shown in FIG. 14. The first waveform (generallydesignated 2000) shown in FIG. 20 is produced by inserting a 20 msecpause (denoted 2002) between the second reset pulse and the set pulse ofthe waveform, with the two reset pulses shifted 20 msec earlier in time,and with a corresponding reduction in the period of zero voltage at thebeginning of the waveform. The second waveform (generally designated2020) shown in FIG. 20 is generally similar to waveform 2000 butwaveform 2020 has its pause (denoted 2022) inserted 40 msec later thanpause 2002, after the first four scan frames of the set pulse. The thirdwaveform (generally designated 2040) shown in FIG. 20 is also generallysimilar to waveform 2000 but waveform 2040 has its pause (denoted 2042)inserted 130 msec later than pause 2002, after the first thirteen scanframes of the set pulse. In both waveforms 2020 and 2040, the scanframes of the set pulse preceding the pause 2022 or 2042 respectivelyare moved earlier in time by 20 msec, as compared with waveform 2000, toaccommodate the pause. As already mentioned, the final gray levelachieved by the waveform is sensitive to both the presence and thelocation of pauses, so modifications of a base waveform such as thoseshown in FIG. 20 can be used to fine tune the final gray level producedby the waveform.

As already noted, it is desirable that IMP drive schemes be DC balanced,in the sense that for any gray level loop (i.e., any sequence of graylevels beginning and ending at the same gray level), the algebraic sumof the impulses applied to a pixel is zero. Example of gray level loopsare:

-   -   1→1    -   2→3→2    -   4→4→3→2→4.        As discussed in copending Application Ser. No. 60/595,729, filed        Aug. 1, 2005 (the entire disclosure of which is herein        incorporated by reference), one can define an irreducible gray        level loop as a sequence of gray levels, starting at a first        gray level, passing through zero or more gray levels to end up        at the first gray level, and not visiting any gray level more        than once, except for the final gray level, which as already        noted must be the same as the first. Obviously, for any gray        scale, there are a finite number of irreducible loops.        Furthermore, it can be shown that any sequence of gray levels,        for example the complex sequence:    -   1→4→3→2→3→2→3→2→1→2→1        can be reduced to sequences of irreducible loops and irreducible        loops embedded within irreducible loops. For example, the above        sequence can be decomposed into a finite set of irreducible        loops, namely two consecutive 2→3→2 loops embedded into a        1→4→3→2→1 loop, and followed by the loop 1→2→1.

If all irreducible loops are DC balanced, all possible sequences thatstart and end at the same gray level are DC balanced. The preferredembodiment of the IMP drive scheme is one in which the net voltageimpulses for all irreducible loops are zero, that is, the waveform is DCbalanced.

It is not absolutely necessary to DC balance an IMP waveform. Whilelarge DC imbalances cause the imaging performance of the display tosuffer, small amounts of DC imbalance can be acceptable. When it is notpossible to achieve complete DC balancing, IMP drive schemes aredesirably controlled so that the net impulse of any irreducible loopdivided by the number of transitions in that loop is less than Q, whereQ is one fourth of the lesser of the absolute values of the net impulsesfor transitions between the two extreme optical states of a pixel, wherethe impulse is determined using a characteristic voltage of the drivescheme. The net impulse required to drive the imaging film from oneextreme optical state to the other represents a characteristic impulseof a medium and near DC imbalance should be measured relative to thischaracteristic impulse.

It is also often desirable that an IMP drive scheme be of the “picketfence” type. As described in the aforementioned MEDEOD applications, itis often necessary or desirable to drive electro-optic displays usingdrive circuitry which can supply only two drive voltages. Since bistableelectro-optic media normally need to be driven in both directionsbetween their extreme optical states, it might at first appear that atleast three drive voltages would be required, namely 0, +V and −V, whereV is an essentially arbitrary drive voltage, so that one electrode for aspecific pixel (typically the common front electrode in a conventionalactive matrix display) could be held at 0, while the other electrode(typically the pixel electrode for that pixel) can be held at +V or −Vdepending upon the direction in which the pixel needs to be driven. Whentwo-voltage drive circuitry is used, each waveform of a drive scheme isdivided into time segments; typically these time segments are of equalduration, but this is not necessarily the case. In a non-picket fencedrive scheme, there may be applied to any specific pixel, in any timesegment, a positive, zero or negative driving voltage. For example, in athree drive voltage system, the common front electrode may be held at 0,while the individual pixel electrodes are held at +V, 0 or −V. In apicket fence drive scheme, each time segment is in effect divided intotwo; in one of the two resultant segments, there may be applied to anyspecific pixel only a negative or zero driving voltage, while in theother resultant segment, there may be applied to any specific pixel onlya positive or zero driving voltage. For example, consider a two drivingvoltage system having driving voltages V and v, where V>v. In the firstof each pair of segments, the common front electrode is set to V, andthe pixel electrodes to either V (zero driving voltage) or v (negativedriving voltage). In the second of each pair of segments, the commonfront electrode is set to v, and the pixel electrodes to either v (zerodriving voltage) or V (positive driving voltage). The resultant waveformis twice as long as the corresponding non-picket fence waveform.

It is also often desirable that an IMP drive scheme be capable of localupdates. As described in the aforementioned MEDEOD applications, it isoften desirable to drive electro-optic displays in a manner whichpermits local updating of a specific area of the display which isundergoing changes while the rest of the display remains unchanged; forexample, it may be desirable to update a dialogue box in which a user isentering text without updating the background image on the display. Alocal update version of any IMP drive scheme can be created by removingall non-zero voltages from the waveforms for zero transitions (i.e.,transitions from one gray level to the same gray level). For example,the waveform from gray level 2 to gray level 2 normally is composed of aseries of voltage pulses. Removing the non-zero voltages from thiswaveform, and doing so for all other zero transitions, results in alocal update version of the IMP waveform. Such a local update versioncan be advantageous when it is desired to minimize extraneous flashingduring transitions.

The following experiments illustrate the use of the modificationsdiscussed above in fine control of gray levels produced by an IMP drivescheme.

An encapsulated electrophoretic medium comprising an internal phase,comprising polymer-coated titania and polymer-coated carbon blackparticles in a hydrocarbon liquid, encapsulated in gelatin/acaciacapsules, was prepared and incorporated into experimental single-pixeldisplays, all substantially as described in Paragraphs [0069] to [0076]of the aforementioned 2002/0180687. The experimental displays were thendriven using a four gray level IMP drive scheme. It was found that thedisplays could driven from gray level 4 (white) to gray level 1 (black)by a +15V, 500 msec pulse, and the reverse transition effected by a−15V, 500 msec pulse, and a basic two reset pulse IMP drive scheme wasconstructed accordingly. FIG. 21 of the accompanying drawings shows, ina highly schematic manner, all sixteen waveforms of this basic IMP drivescheme, which are labeled as [R1 R2] so that the first number givenrepresents the final gray state. For example, the [1 4] waveform shownin the upper right hand corner of FIG. 21 effects the transition fromgray level 4 (white) to gray level 1 (black) and comprises a first +15 V500 msec reset pulse, which drives the pixel black, a second −15 V 500msec reset pulse, which drives the pixel white, and a +15 V 500 msec setpulse, which drives the pixel black.

An experimental display was driven using this basic IMP drive schemethrough varying sequences of gray levels and the reflectance of thedisplay measured at the conclusion of each sequence; the results areshown in FIG. 22. Each point in FIG. 22 represents a reflectancefollowing a different sequence of gray levels prior to reaching thefinal gray level shown on the abscissa. It will be seen from FIG. 22that the reflectances achieved at the same nominal gray level variedconsiderably, and such variation if of course undesirable since itadversely affects the quality of the image produced by a multi-pixeldisplay. In particular, the human eye is very sensitive to minorvariations in gray level occurring within a block of pixels which aresupposed to be at the same gray level, and FIG. 22 indicates that suchvariation could be expected as a result of differences in the prior graylevels of the pixels.

The IMP drive scheme was then modified in the manner described above byinsertion and excision of balanced pulse pairs (with closing up of theresultant gaps in the case of excision) and insertion or removal ofperiods of zero voltage at the beginning or end of various waveforms, inorder to achieve consistent gray levels after various gray levelsequences, to produce the modified IMP drive scheme shown in FIG. 23.FIG. 24 shows the gray levels produced by the modified IMP drive schemeof FIG. 23 using the same gray level sequences as in FIG. 22. It will beseen from FIG. 24 that the modified IMP drive scheme of FIG. 24 producedmuch more consistent gray levels than the unmodified drive scheme ofFIG. 21.

Section C: Balanced Pulse Pair Dwell Time Compensation Method andApparatus

As already mentioned, in a third aspect, this invention provides amethod for driving an electro-optic display having at least one pixelcapable of achieving at least two different gray levels. In this method,at least two different waveforms are used for the same transitionbetween specific gray levels depending upon the duration of the dwelltime of the pixel in the state from which the transition begins; thesetwo waveforms differ from each other by at least one insertion and/orexcision of at least one balanced pulse pair, or insertion of at leastone period of zero voltage, where “balanced pulse pair” has the meaningpreviously defined. It is very much preferred that in such a method thedrive scheme be DC balanced as that term has been defined above.

In such a balanced pulse pair dwell time compensation (BPPDTC) method(as in the BPPSS and IMP methods already described), the insertion orexcision of the balanced pulse pair and/or of the zero voltage period(pause) may be effected either within a single waveform element orbetween two successive waveform elements. The two waveforms used for thesame transition following differing dwell times in the initial statefrom which the transition begins may be referred to hereinafter as the“alternative dwell time” or “ADT” waveforms.

It should be noted that ADT waveforms may differ from one another by thelocation and/or duration of a BPP or pause within a waveform (see, forexample, the discussion of FIGS. 25B-25E below), since such movement ofa BPP or pause may be formally regarded as a combination of an excisionof a BPP or pause at one location and an insertion of the BPP or pauseat a different location, or (in the case of a change of duration at thesame location) as a combination of an excision of a BPP or pause at thelocation and an insertion of a different BPP or pause at the samelocation.

In a BPPDTC drive scheme, the insertion of excision of BPP's and/orpauses raises the same problems, and may be handled in the same way, asin the BPPSS and modified IMP drive schemes described in Sections A andB above. Thus, where the difference between the ADT waveforms inaccordance with the BPPDTC aspect of the present invention includesexcision of at least one BPP, the period formerly occupied by the oreach excised BPP may be left as a period of zero voltage. Alternatively,this period may be “closed up” by moving some or all of the laterwaveform elements earlier in time, normally with insertion of a periodof zero voltage at some later stage in the waveform, typically at theend thereof, in order to ensure that the overall length of the waveformis maintained. (In any practical display, which will normally have atleast several thousand pixels, in any transition there will normally beat least one pixel undergoing every possible transition, and if thewaveforms for all pixels are not of the same length, controller logicbecomes extremely complicated.) Alternatively, of course, the period maybe “closed up” by moving some or all of the earlier waveforms elementslater in time, with insertion of a period of zero voltage at someearlier stage of the waveform, typically at the beginning thereof.

Similarly, inserting a BPP adds to the total duration of a waveformunless an existing period of zero voltage can simultaneously be removed.Since all waveforms of a drive scheme very desirably have the sameoverall length, when one waveform of a drive scheme has a BPP inserted,all the other waveforms of the drive scheme should have a period of zerovoltage added to them, or some other modification made, to compensatefor the increase in overall waveform length caused by the insertion ofthe BPP. For example, if a 40 msec BPP is inserted into theblack-to-white waveform shown in Table 1 above (which has a waveformlength of 420 msec), 40 msec pauses could be added to the remainingthree waveforms shown in Table 1 so that all the waveforms have a lengthof 460 msec. Obviously, if appropriate, BPP's could be added to theother three waveforms rather than pauses, or some combination of BPP'sand pauses totaling 40 msec could be used.

Preferred drive schemes and waveforms of the BPPDTC aspect of thepresent invention will now be described, though by way of illustrationonly. The balanced pulse pairs used in such drive schemes and waveformsmay be of any of the types described above; for example, the types ofBPP's shown in FIGS. 4A-4C may be used.

FIGS. 25A-25E illustrate alternative dwell time waveforms which may beused for a single transition in accordance with the BPPDTC aspect of thepresent invention. FIG. 25A illustrates the black-to-white waveformmentioned in the third line of Table 1 and the last line of Table 2above. Since this is the waveform appropriate for the black-to-whitetransition after a long dwell time in the black state, it may beregarded as the base black-to-white waveform which is modified inaccordance with the BPPDTC aspect of the present invention to producewaveforms appropriate for the black-to-white transition after shorterdwell times in the black state. As already noted, the base waveform ofFIG. 25A consists of a −15V, 400 msec pulse followed by 0 V for 20 msec.

FIG. 25B illustrates a modification of the base waveform of FIG. 25Awhich has been found effective to decrease the reflectance of the finalwhite state when a black-to-white transition is effected after only ashort dwell time of not more than 0.3 seconds in the initial blackstate. The waveform of FIG. 25B is produced by inserting a BPP similarto BPP 300 shown in FIG. 4A at the end of the −15V, 400 msec pulse ofthe waveform of FIG. 25A, so that the waveform of FIG. 25B comprises a−15V, 420 msec pulse, followed by a +15V, 20 msec pulse and 0 V for 20msec.

FIGS. 25C and 25D illustrate two further ADT waveforms for the sameblack-to-white transition as the waveforms of FIGS. 25A and 25B. Thewaveforms of FIGS. 25C and 25D have been found effective to standardizethe reflectance of the final white state when the black-to-whitetransition is effected after dwell times of 0.3 to 1 second, and 1 to 3seconds, respectively, in the black state. The waveforms of FIGS. 25Cand 25D are produced by inserting the same BPP as in FIG. 25B into thewaveform of FIG. 25A, but at locations different from that used in FIG.25B. As noted above, it has been found that the position at which a BPPis inserted into (or excised from) a base waveform has a significanteffect on the final optical state following a transition, and hence thatshifting the position of insertion of a BPP with a base waveform is aneffective means for compensating the waveform for variations in thedwell time of the pixel in the initial optical state.

FIG. 25E is a preferred alternative to the waveform of FIG. 25A foreffecting the black-to-white transition after long dwell times (3seconds or greater) in the black state. The waveform of FIG. 25E isgenerally similar to those of FIGS. 25B-25D in that it is produced byinserting the same BPP into the waveform of FIG. 25A. However, in FIG.25E, the BPP is inserted at the beginning of the waveform; it has alsobeen found desirable to make the pulses of the BPP 40 msec rather than20 msec in duration. Since this makes the overall duration of thewaveform 500 msec, when the waveform of FIG. 25E is used in conjunctionwith the waveforms of FIGS. 25B-25D, it is necessary to “pad” thewaveforms of FIGS. 25B-25D with an additional 40 msec of 0 V at the endof the waveform. Thus, a preferred set of ADT waveforms for theblack-to-white transition is as shown in Table 3 below:

TABLE 3 Dwell time Waveform 0 to 0.3 s −15 V for 420 ms, 15 V for 20 ms,0 V for 60 ms (FIG. 25B, padded) 0.3 s to 1 s −15 V for 220 ms, 15 V for20 ms, −15 V for 200 (FIG. 25C, padded) ms, 0 V for 60 ms 1 s to 3 s −15V for 20 ms, 15 V for 20 ms, −15 V for 400 (FIG. 25D, padded) ms, 0 Vfor 60 ms 3 s or greater −15 V for 40 ms, 15 V for 40 ms, −15 V for 400(FIG. 25E) ms, 0 V for 20 ms

Note that the impulse for the black-to-white transition is −15V*400msec, or 6 V sec for all the ADT waveforms in Table 3, and thus for allinitial state dwell times, so that the drive scheme is DC balanced.

As already mentioned, DTC can also be effected by excising BPP's from abase waveform. For example, consider the drive scheme shown in Table 4below:

TABLE 4 Transition Waveform black to black 0 V for 820 ms black to white+15 V for 400 ms, −15 V for 400 ms, then 0 V for 20 ms white to black−15 V for 400 ms, +15 V for 400 ms, then 0 V for 20 ms white to white 0V for 820 ms

Note that, in this drive scheme, not merely the whole drive scheme butall waveforms are “internally” DC balanced; the desirability of suchinternal DC balancing is discussed in detail in the aforementionedcopending application Ser. No. 10/814,205. Again, the method for DTCwill be discussed with reference to the black-to-white transition,although it should be understood that DTC of the white-to-blacktransition can be effected in a similar manner.

In this case, DTC of the black-to-white transition is effected byexcising BPP's, i.e., by removing a portion of one voltage pulse of onepolarity and one duration while simultaneously removing a similarportion of one voltage pulse of the opposite polarity and equivalentduration. One can either replace the pulse sections that were excisedwith a period of zero voltage or the remaining parts of the waveform canbe shifted in time to occupy the period previously occupied by theexcised pulse pair, and, in order to maintain the total update time, azero voltage segment matching the duration of the excised pair can beadded elsewhere, typically at the beginning or end of the waveform.

FIGS. 26A, 26B and 26C illustrate schematically this process formodification of the black-to-white waveform listed in the third row ofTable 4 above for DTC at short dwell times of less than 0.3 seconds inthe black state. FIG. 26A illustrates the base waveform from Table 4.FIG. 26B shows schematically excision of a BPP formed by the last 80msec portion of the positive voltage pulse and the first 80 msec portionof the negative voltage pulse from the waveform of FIG. 26A, with theresultant gap being eliminated by shifting the negative pulse forward intime, as indicated by the arrow in FIG. 26B. The resultant dwell timecompensated waveform, which comprises a 320 msec positive pulse, a 320msec negative pulse and a 180 msec period of zero voltage, is shown inFIG. 26C.

In this case, it was found that DTC for all dwell times could beeffected simply by varying the length of the excised BPP, and that forlong dwell times of 3 seconds or more in the black state the basewaveform of FIG. 26A was satisfactory. Hence the full list of ADTwaveforms for the black-to-white transition in this case is as shown inTable 5 below:

TABLE 5 Dwell time Waveform 0 to 0.3 s +15 V for 320 ms, −15 V for 320ms, then 0 V for 180 ms 0.3 s to 1 s +15 V for 360 ms, −15 V for 360 ms,then 0 V for 100 ms 1 s to 3 s +15 V for 380 ms, −15 V for 380 ms, then0 V for 60 ms 3 s or greater +15 V for 400 ms, −15 V for 400 ms, then 0V for 20 ms

As already mentioned, when a BPP is excised from a base waveform in themanner shown in FIG. 26B, it is not essential that the remainingcomponents be shifted in time; the excised BPP can simply be replaced bya period of zero voltage. Table 6 below shows a modified set of ADTwaveforms similar to those in Table 5 but with the excised BPP'sreplaced with periods of zero voltage:

TABLE 6 Dwell time Waveform 0 to 0.3 s +15 V for 320 ms, 0 V for 160 ms,−15 V for 320 ms, then 0 V for 20 ms 0.3s to 1 s +15 V for 360 ms, 0 Vfor 80 ms, −15 V for 360 ms, then 0 V for 20 ms 1 s to 3 s +15 V for 380ms, 0 V for 40 ms, −15 V for 380 ms, then 0 V for 60 ms 3 s or greater+15 V for 400 ms, −15 V for 400 ms, then 0 V for 20 ms

Although the BPPDTC aspect of the present invention has been describedabove primarily with reference to displays having only two gray levels,it is not so limited but may be applied to displays having a greaternumber of gray levels. Also, although in the specific waveformsillustrated in the drawings, insertion or excision of the two elementsof a BPP has been effected at a single point within the waveform, theinvention is not limited to waveforms in which insertion or excision ofa BPP is effected at a single point; the two elements of a BPP may beinserted or excised at different points, i.e., the two pulses that makeup a BPP do not have to be immediately sequential, but could beseparated by a time interval. Furthermore, one or both pulses of a BPPcould be subdivided into sections and these sections could be theninserted into or excised from a waveform for DTC. For example, a BPP maybe composed of a +15 V, 60 msec pulse and a −15 V, 60 msec pulse. ThisBPP could be divided into two components, for example a +15 V, 60 msecpulse followed immediately by a −15 V, 20 msec pulse, and a −15 V, 40msec pulse, and these two components simultaneously inserted into orexcised from a waveform to achieve DTC.

Inserting or excising zero voltage segments from a waveform has alsobeen found to affect the final gray level after a transition, and hencesuch insertion or excision of zero voltage segments provides a secondmethod for tuning the final gray level to achieve DTC. Such insertion orexcision of zero voltage segments may be used alone or in combinationwith insertion or excision of BPP's.

Although the BPPDTC aspect of the present invention has been describedabove primarily with reference to pulse width modulated waveforms inwhich the voltage applied to a pixel at any given time can only be −V, 0or +V, the invention is not limited to use with such pulse widthmodulated waveforms and may be used with voltage modulated waveforms, orwaveforms using both pulse and voltage modulation. The foregoingdefinition of a balanced pulse pair can be satisfied by two pulses ofopposite polarity with zero net impulse, and does not require that thetwo pulses be of the same voltage or duration. For example, in a voltagemodulated drive scheme, a BPP might be composed of a +15 V, 20 msecpulse followed by a −5 V, 60 msec pulse.

From the foregoing, it will be seen that the BPPDTC aspect of thepresent invention permits dwell time compensation of a drive schemewhile maintaining DC balance of the drive scheme. Such DTC can reducethe level of ghosting in electro-optic displays.

Section D: Target Buffer Methods and Apparatus

As already mentioned, the present invention provides two differentmethods using target buffers for driving electro-optic displays having aplurality of pixels, each of which is capable of achieving at least twodifferent gray levels. The first of these two methods, the non-polaritytarget buffer method comprises providing initial, final and target databuffers; determining when the data in the initial and final data buffersdiffer, and when such a difference is found updating the values in thetarget data buffer in such a manner that (i) when the initial and finaldata buffers contain the same value for a specific pixel, setting thetarget data buffer to this value; (ii) when the initial data buffercontains a larger value for a specific pixel than the final data buffer,setting the target data buffer to the value of the initial data bufferplus an increment; and (iii) when the initial data buffer contains asmaller value for a specific pixel than the final data buffer, settingthe target data buffer to the value of the initial data buffer minussaid increment; updating the image on the display using the data in theinitial data buffer and the target data buffer as the initial and finalstates of each pixel respectively; next, copying the data from thetarget data buffer into the initial data buffer; and these steps untilthe initial and final data buffers contain the same data.

In the second of these two methods, the polarity target buffer method,the final, initial and target data buffers are again provided, togetherwith a polarity bit array arranged to store a polarity bit for eachpixel of the display. Again, the data in the initial and final databuffers are compared, and when they differ the values in the polaritybit array and target data buffer are updated in such a manner that (i)when the values for a specific pixel in the initial and final databuffers differ and the value in the initial data buffer represents anextreme optical state of the pixel, the polarity bit for the pixel isset to a value representing a transition towards the opposite extremeoptical state; and the target data buffer is set to the value of theinitial data buffer plus or minus an increment, depending upon therelevant value in the polarity bit array. The image on the display isthen updated in the same way as in the first method and thereafter thedata from the target data buffer is copied into the initial data buffer.These steps are repeated until the initial and final data bufferscontain the same data.

Prior art controllers for bistable electro-optic displays typically uselogic similar to that shown in the following Listing 1 (all Listingsherein are in pseudocode):

Listing 1 pixel array initial [x_size, y_size] pixel array final[x_size, y_size] while( )#endless loop initial := final if (host has newdata) final := new_image update_display (initial, final)

With a controller operating in this manner, the display waits to receivenew image information, then, when such new image information isreceived, performs a full update before allowing new information to besent to the display, i.e., once one new image has been accepted by thedisplay, the display cannot accept a second new image until therewriting of the display needed to display the first new image has beencompleted, and in some cases this rewriting procedure may take hundredsof milliseconds cf. some of the drive schemes set out in Sections A-Cabove. Therefore, when the user is scrolling or typing, the displayappears insensitive to user input for this full update (rewriting) time.

In contrast, a controller effecting the non-polarity target buffermethod of the present invention operates by logic exemplified by thefollowing Listing 2 (this type of controller may hereafter forconvenience be called a “Listing 2 controller”):

Listing 2 pixel array initial [x_size, y_size] pixel array final[x_size, y_size] pixel array target [x_size, y_size] while( )#endlessloop initial := target final := host_frame_buffer if initial != finalfor each pixel in initial if Initial == final then target := initial ifInitial > final then target := initial + 1 if initial < final thentarget := initial −1 update_display (initial, target)

In this modified controller logic for an NPTB method, there are threeimage buffers. The initial and final buffers are the same as in priorart controllers, and the new third buffer is a “target” buffer. Thedisplay controller can accept new image data at any time into the finalbuffer. When the controller finds that the data in the final buffer isno longer equal to the data in the initial buffer (i.e., rewriting ofthe image is required), a new target data set is constructed byincrementing or decrementing the values in the initial buffer by one (orleaving them unchanged), depending upon the difference between therelevant values in the initial and final buffers. The controller thenperforms a display update in the usual way using the values from theinitial and target buffers. When this update is complete, the controllercopies the values from the target buffer into the initial buffer, andthen repeats the differencing operation between the initial and finalbuffers to generate a new target buffer. The overall update is completewhen the initial and final buffers have the same data set.

Thus, in this NPTB method, the overall update is effected as a series ofsub-update operations, one such sub-update operation occurring when theimage is updated using the initial and target buffers. The term“meso-frame” will be used hereinafter for the period required for eachof these sub-update operations; such a meso-frame of course designates aperiod between that required for a single scan frame of the display (cf.the aforementioned MEDEOD applications) and the superframe, or periodrequired to complete the entire update.

The NPTB method of the present invention improves interactiveperformance in two ways. Firstly, in the prior art method, the finaldata buffer is used by the controller during the update process, so thatno new data can be written into this final data buffer while an updateis taking place, and hence the display is unable to respond to new inputduring the entire period required for an update. In the NPTB method ofthe present invention, the final data buffer is used only forcalculation of the data set in the target data buffer, and thiscalculation, being simply a computer calculation, can be effected muchmore rapidly than the update operation, which requires a physicalresponse from the electro-optic material. Once the calculation of thedata set in the target data buffer is complete, the update does notrequire further access to the final data buffer, so that the final databuffer is available to accept new data.

For reasons discussed in the aforementioned MEDEOD applications andfurther discussed below with regard to waveforms, it is often desirablethat pixels be driven in a cyclic manner, in the sense that once a pixelhas been driven from away from one extreme optical state by a voltagepulse of one polarity, no voltage pulse of the opposite polarity isapplied to that pixel until the pixel reaches its other extreme opticalstate; see, for example FIGS. 11A and 11B and the related description ofthe aforementioned 2003/0137521. This restriction is satisfied by thePTB method of the present invention, which may use a controlleroperating with logic exemplified by the following Listing 3 (this typeof controller may hereafter for convenience be called a “Listing 3controller”; this Listing assumes a four gray level system with graylevels numbered from 1 for black to 4 for white, although those skilledin the art can readily modify the pseudocode for operation withdiffering numbers of gray levels):

Listing 3 pixel array initial [x_size, y_size] pixel array final[x_size, y_size] pixel array target [x_size, y_size] bit array polarity[x_size, y_size] while( )#endless loop initial := target final :=host_frame_buffer if initial != final for each pixel in initial ifinitial == 1 then polarity := 1 if initial == 4 then polarity := 0 ifinitial != final then target := initial + (polarity−0.5)*2update_display (initial, target)

This PTB method requires four image buffers, the fourth being a“polarity” buffer having a single bit for each pixel of the display,this single bit indicating the current direction of transition of theassociated pixel, i.e., whether the pixel is currently transitioningfrom white-to-black (0) or black-to-white (1). If the associated pixelis not currently undergoing a transition, the polarity bit retains itsvalue from the previous transition; for example, a pixel that isstationary in a light gray state and was previously white will have apolarity bit of 0.

In the PTB method, the polarity bit array is taken into account when anew target buffer data set is constructed. If the pixel is currentlyblack or white, and a transition to the opposite state is required, thevalue of the polarity bit is set accordingly, and the target value isset to the gray level closest to black or white respectively.Alternatively, if the initial state for the pixel is an intermediate(gray) state, the target value is calculated by incrementing ordecrementing the state by 1, according to the value of the polarity bit(+1 if polarity=1; −1 if polarity=0).

It should be noted that, in this drive scheme, the behavior of pixels inthe intermediate states is independent of the current value of the finalstate for that pixel. A pixel, upon starting a transition from black towhite or white to black, will continue in the same direction until itreaches the opposite optical rail (extreme optical state, typicallyblack or white). If the desired image and hence the target state changesduring the transition, the pixel will then return in the oppositedirection, and so on.

Preferred waveforms for use in TB methods of the present invention willnow be discussed. Table 7 below illustrates one possible transitionmatrix which can be used for one-bit (monochrome) operation with NPTBand PTB methods of the present invention, this transition matrix usingtwo intermediate states.

TABLE 7 Initial State 1 2 3 4 Target 1 A(*) B — — State 2 C — E — 3 — F— H 4 — — I J(*)

The structure of this transition matrix, with black, white, and twointermediate gray states, looks very similar to those used in prior arttwo-bit drive schemes, such as those described in the MEDEODapplications. However, in the TB methods of the present invention, theseintermediate states do not correspond to stable gray states, but areonly transition states, which exist only between the completion of onemeso-frame and the start of the next. Also, there is no restriction onthe uniformity of the reflectivity of these intermediate states.

It should be noted that, in the transition matrix shown in Table 7, manyof the elements (indicated by the dashes) are not allowed. Thecontroller only allows each transition to change the gray level by oneunit in either direction, so that transitions involving multiple changesin gray level (for example a direct 1-4 black-to-white transition) areforbidden. The elements on the leading diagonal of the transition matrix(corresponding to zero transitions) are forbidden for the intermediatestates; such leading diagonal elements are not recommended for white andblack states, but are not strictly forbidden, as indicated by theasterisks in Table 7.

In a monochrome NPTB method, an update sequence appears as a series ofstates, starting and ending at the extreme optical states (opticalrails), with a sequence of intermediate gray states separated by zerodwell time. For example, a simple transition from black to white wouldappears as:

-   -   1        2        3        4        On the other hand, if the final state of the display changes        during the update, this transition might become:    -   1        2        3        2        1        Multiple changes in the final state might produce transitions        such as:    -   1        2        3        2        3        4        More generally, there are four possible types of transitions        between the extreme black and white optical states:    -   1        2        3(        2        3)        4    -   1        2(        3        2)        1    -   4        3        2(        3        2)        1    -   4        3(        2        3)        4        where the parentheses signify zero or more repeats of the        sequence within the parentheses.

Optimization (“tuning”) of this class of NPTB drive schemes requiresadjusting the non-zero elements of the transition matrix to ensureconsistent reflectivity values for the 1 (black) and 4 (white) states,independent of the number of repeats of the parenthetical sequences. Thewaveform must work for arbitrary dwell times in the black and whiteextreme optical states, but the dwell times in the intermediate statesare always zero, so that, as mentioned above, the reflectivities of thetransition states are not important.

In general, the time required for any single meso-frame update is equalto the length of the longest element in the transition matrix. Thus, thetime for a total update is three times the length of this longestelement. In the best case, the black-to-white and white-to-black (1

4 and 4

1 respectively) waveforms can be segmented into three equal-lengthpieces; this approach will reduce the update latency to one third of thefull update time, while maintaining the same duration for the fullupdate. As the length of the meso-frame updates becomes longer, whichmay be the result of optimizing the waveform, the benefit becomes lesssubstantial. For example, if one element becomes twice as long, then thelatency increases to two-thirds of the simple update time, and the fulltransition will require twice as long as before. It is possible to testto find the longest element present in a given meso-frame, anddynamically adjust the update time to that length, but the benefit ofthis extra computation is not likely to be significant.

Consideration should be given to what electro-optical properties of amedium make the display using the medium suitable for use with this typeof NPTB drive scheme. Firstly, the dwell time dependency of the mediumshould be zero (ideally, or at least very low), since this waveformcombines a series of near zero dwell times between meso-frames withpotentially much longer dwell times between transitions. Secondly, themedium should have little or no sensitivity to optical states precedingthe initial state of a particular transition, because the direction of atransition may change in mid-stream; for example, a 2

1 transition might be preceded by either a 1

2 or a 3

2 transition. Finally, the electro-optic medium should be symmetric inits response, especially near the black and white states; it isdifficult to produce a DC balanced waveform that can perform a 1

2

1 or 4

3

4 transition that reaches the same black or white state, respectively.

For the foregoing reasons, the “intermediate reversals” in NPTB driveschemes make it very difficult to develop optimized waveforms. Incontrast, a PTB drive scheme greatly reduces the demands on theelectro-optic medium, and hence should alleviate much of the difficultyin optimizing an NTPB drive scheme while still providing improvedperformance.

Although the structure of the transition matrix for a PTB drive schemeis identical to that for an NPTB drive scheme, a PTB drive schemepermits only two black-to-white and white-to-black transitions, namely:

-   -   1        2        3        4; and    -   4        3        2        1.        In fact, these two transitions can be the same as the normal 1        4 and 4        1 transitions, with the transitions partitioned into three equal        parts. Some slight re-tuning may be desirable to account for any        delays between the meso-frames, but the adjustment is        straightforward. For simple typing input, this drive scheme        should result in a two-thirds reduction in latency.

There are some drawbacks to a PTB method. Extra memory is required forthe polarity bit array, and a more complex controller is operate thissimpler drive scheme because allowing for the direction of thetransition at each pixel requires taking account of an extra datum (thepolarity bit) in addition to the initial and final states for atransition. Also, while a PTB method does reduce the latency forstarting an update, the controller must wait until an update is completebefore reversing the transition. This limitation is apparent if a usertypes a character, and then immediately erases it; the delay before thecharacter is erased is equal to the full update time. This limits theusefulness of the PTB method for cursor tracking or scrolling.

Although the NPTB and PTB methods have been described above primarilywith regard to monochrome drive schemes, they are also compatible withgray scale drive schemes. The NPTB method is inherently completely grayscale compatible; the gray scale compatibility of a PTB method isdiscussed below.

From a drive scheme perspective, it will obviously be more difficult toproduce a workable gray scale drive scheme for an NPTB method than acorresponding monochrome drive scheme, because in the gray scale drivescheme the intermediate states now correspond to actual gray levels, andthus the optical values of these intermediate states are constrained.Producing a gray scale drive scheme for a PTB method is also quitedifficult. To reduce latency, the meso-frame transitions must beappreciably shortened. For example, a 2

3 transition could be a stand-alone transition, the last stage of a 1

2

3 transition, or the first stage of a 2

3

4 transition. Thus, there are competing demands to make this transitionshort (to achieve a shorter overall update), and accurate (in case thetransition stops at gray level 3).

A gray scale PTB method may be modified by introducing multiple graylevel steps (i.e., by permitting the gray level to change by more thanone unit during each meso-frame, corresponding to re-inserting elementsmore than one step removed from the leading diagonal of the relevanttransition matrix, such as that shown in Table 7 above), thuseliminating the degeneracy of the meso-frame steps described in thepreceding paragraph. This modification could be effected by replacingthe polarity bit matrix with a counter array, which contains, for eachpixel of the display, more than one bit, up to the number of bitsrequired for a full gray scale image representation. The waveform wouldthen contain up to a full N×N transition matrix, with each waveformdivided evenly into four (or other essentially arbitrary number ofmeso-frames).

Although the specific TB methods discussed above are two-bit gray scalemethods, with two intermediate gray levels, TB methods can of course beused with any number of gray levels. However, the incremental benefit ofreduced latency will tend to decrease as the number of gray levelsgrows.

Thus, the present invention provides two types of TB methods that givesignificant reductions in update latency in monochrome mode, whileminimizing the complexity of the controller algorithms. These methodsmay prove especially useful in interactive one-bit (monochrome)applications, for example, personal digital assistants and electronicdictionaries, where a fast response to user input is of paramountimportance.

Section E: Waveform Compression Methods and Apparatus

As already mentioned, the last main aspect of the present inventionrelates to a method for reducing the amount of waveform data which hasto be stored in order to drive a bistable electro-optic display. Morespecifically, this aspect of the present invention provides a “waveformcompression” or “WC” method for driving an electro-optic display havinga plurality of pixels, each of which is capable of achieving at leasttwo different gray levels, the method comprising: storing a basewaveform defining a sequence of voltages to be applied during a specifictransition by a pixel between gray levels; storing a multiplicationfactor for the specific transition; and effecting the specifictransition by applying to the pixel the sequence of voltages for periodsdependent upon the multiplication factor.

When an impulse-driven electro-optic display is being driven, each pixelof the display receives a voltage pulse (i.e., a voltage differentialbetween the two electrodes associated with that pixel) or temporalseries of voltage pulses (i.e., a waveform) in order to effect atransition from one optical state of the pixel to another, typically atransition between gray levels. The data needed to define the set ofwaveforms (forming a complete drive scheme) for each transition isstored in memory, generally on the display controller, although the datacould alternatively be stored on a host computer or other auxiliarydevice. A drive scheme may comprise a large number of waveforms, and (asdescribed in the aforementioned MEDEOD applications) it may be necessaryto store multiple sets of waveform data to allow for variations inenvironmental parameters such as temperature and humidity, andnon-environmental variations, for example the operating life of theelectro-optic medium. Thus, the amount of memory needed to hold thewaveform data can be substantial. It is desirable to reduce this amountof memory in order to reduce the cost of the display controller. Asimple compression scheme that can be realistically accommodated in adisplay controller or host computer would be helpful in reducing theamount of memory needed for waveform data and thus the displaycontroller cost. The waveform compression method of the presentinvention provides a simple compression scheme that is particularlyadvantageous for electrophoretic displays and other known bistabledisplays.

Uncompressed waveform data for a particular transition is typicallystored as a series of bit sets, each bit set specifying a particularvoltage to be applied at a particular point in the waveform. By way ofexample, consider a tri-level voltage drive scheme, where a pixel isdriven toward black using a positive voltage (in this example, +10 V),toward white using a negative voltage (−10 V), and held at its currentoptical state with zero voltage. The voltage for a given time element (ascan frame for an active matrix display) can be encoded using two bits,for example, as shown in Table 8 below:

TABLE 8 Desired voltage (V) Binary representation +10 01 −10 10 0 00

Using this binary representation, a waveform for use in an active matrixdrive and comprising a +10V pulse lasting for five scan frames followedby two scan frames of zero voltage would be represented as:

-   -   01 01 01 01 01 00 00.        Waveforms that comprise a large number of time segments require        the storage of a large number of bit sets of waveform data.

In accordance with the WC method of the present invention, waveform datais stored as a base waveform (such a binary representation describedabove) and a multiplication factor. The display controller (or otherappropriate hardware) applies to a pixel the sequence of voltagesdefined by the base waveform for periods dependent upon themultiplication factor. In a preferred form of such a WC method, a bitset (such as that given above) is used to represent the base waveform,but the voltage defined by each bit set is applied to the pixel for ntime segments, where n is the multiplication factor associated with thewaveform. The multiplication factor must be a natural number. For amultiplication factor of 1, the waveform applied is unchanged from thebase waveform. For a multiplication factor greater than 1, therepresentation of the voltage series is compressed for at least somewaveforms, that is, fewer bits are needed to express these waveformsthan would be needed if the data were stored in uncompressed form.

By way of example, using the three voltage level binary representationof Table 8, consider a waveform that requires twelve scan frames of +10Vfollowed by nine scan frames of −10V followed by six scan frames of +10Vfollowed by three scan frames of 0V. This waveform is expressed inuncompressed form as:

-   -   01 01 01 01 01 01 01 01 01 01 01 01 10 10 10 10 10 10 10 10 10        01 01 01 01 01 01 00 00 00        and in compressed form as:    -   multiplication factor: 3    -   base waveform 01 01 01 01 10 10 10 01 01 00.

The length of the voltage sequence that must be allocated for eachwaveform is determined by the longest waveform. For encapsulatedelectrophoretic and many other electro-optic displays, the longestwaveforms are typically required at the lowest temperatures, where theelectro-optic medium responds slowly to the applied field. At the sametime, the resolution necessary to achieve successful transitions isreduced when the response is slow, so there is little loss in accuracyof optical state by grouping successive scan frames through the WCmethod of the present invention. Using this compression method, a numberof scan frames (or generally time segments) that is appropriate forwaveforms at moderate and high temperatures where the update time isshort can be allocated to each waveform. At low temperature, where thenumber of scan frames needed can exceed the memory allocation,multiplication factors greater than unity can be used to generate longwaveforms. This ultimately results in reduced memory requirements andcosts.

The WC method of the present invention is in principle equivalent tosimply changing the frame time of an active matrix display at varioustemperatures. For example, a display could be driven at 50 Hz at roomtemperature, and at 25 Hz at 0° C. to extend the allowable waveformtime. However, the WC method is superior because backplanes are designedto minimize the impact of capacitive and resistive voltage artifacts ata given scan rate. As one deviates significantly from this optimum scanrate in either direction, artifacts of at least one type rise. It istherefore better to keep the actual scan rate constant, while groupingscan frames using the WC method, which, in effect, provides a way ofachieving a virtual change in scan rate without actually changing thephysical scan rate.

It will be apparent to those skilled in the art that numerous changescan be made in the specific embodiments of the present invention alreadydescribed without departing from the spirit scope of the invention.Accordingly, the whole of the foregoing description is to be construedin an illustrative and not in a limitative sense.

The invention claimed is:
 1. A method for driving an electro-opticdisplay having a plurality of pixels, each of which is capable ofachieving at least two different gray levels, the method comprising: (a)providing a final data buffer arranged to receive data defining adesired final state of each pixel of the display; (b) providing aninitial data buffer arranged to store data defining an initial state ofeach pixel of the display; (c) providing a target data buffer arrangedto store data defining a target state of each pixel of the display; (d)determining when the data in the initial and final data buffers differ,and when such a difference is found updating the values in the targetdata buffer by (i) when the initial and final data buffers contain thesame value for a specific pixel, setting the target data buffer to thisvalue; (ii) when the initial data buffer contains a larger value for aspecific pixel than the final data buffer, setting the target databuffer to the value of the initial data buffer plus an increment; and(iii) when the initial data buffer contains a smaller value for aspecific pixel than the final data buffer, setting the target databuffer to the value of the initial data buffer minus said increment; (e)updating the image on the display using the data in the initial databuffer and the target data buffer as the initial and final states ofeach pixel respectively; (f) after step (e), copying the data from thetarget data buffer into the initial data buffer; and (g) repeating steps(d) to (f) until the initial and final data buffers contain the samedata.
 2. A method according to claim 1 wherein the display comprises arotating bichromal member or electrochromic medium.
 3. A methodaccording to claim 1 wherein the display comprises an electrophoreticelectro-optic medium comprising a plurality of electrically chargedparticles in a fluid and capable of moving through the fluid onapplication of an electric field to the fluid.
 4. A method according toclaim 3 wherein the fluid is gaseous.
 5. A method according to claim 3wherein the charged particles and the fluid are confined within aplurality of capsules or microcells.
 6. A display controller orapplication specific integrated circuit arranged to carry out the methodof claim
 1. 7. A method for driving an electro-optic display having aplurality of pixels, each of which is capable of achieving at leastthree different gray levels, the method comprising: (a) providing afinal data buffer arranged to receive data defining a desired finalstate of each pixel of the display; (b) providing an initial data bufferarranged to store data defining an initial state of each pixel of thedisplay; (c) providing a target data buffer arranged to store datadefining a target state of each pixel of the display; (d) providing apolarity bit array arranged to store a polarity bit for each pixel ofthe display; (e) determining when the data in the initial and final databuffers differ, and when such a difference is found updating the valuesin the polarity bit array and target data buffer by (i) when the valuesfor a specific pixel in the initial and final data buffers differ andthe value in the initial data buffer represents an extreme optical stateof the pixel, setting the polarity bit for the pixel to a valuerepresenting a transition towards the opposite extreme optical state;and (ii) when the values for a specific pixel in the initial and finaldata buffers differ, setting the target data buffer to the value of theinitial data buffer plus or minus an increment, depending upon therelevant value in the polarity bit array; (f) updating the image on thedisplay using the data in the initial data buffer and the target databuffer as the initial and final states of each pixel respectively; (g)after step (f), copying the data from the target data buffer into theinitial data buffer; and (h) repeating steps (e) to (g) until theinitial and final data buffers contain the same data.
 8. A methodaccording to claim 7 wherein the display comprises a rotating bichromalmember or electrochromic medium.
 9. A method according to claim 7wherein the display comprises an electrophoretic electro-optic mediumcomprising a plurality of electrically charged particles in a fluid andcapable of moving through the fluid on application of an electric fieldto the fluid.
 10. A method according to claim 9 wherein the fluid isgaseous.
 11. A method according to claim 9 wherein the charged particlesand the fluid are confined within a plurality of capsules or microcells.12. A display controller or application specific integrated circuitarranged to carry out the method of claim 7.